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Light and Optics | General Science

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Light and Optics | General Science

Light and Optics

shape Description

Light is an energy carried in an electromagnetic wave emitted by vibrating electrons in the atoms. 'Light is an electromagnetic radiation that has a wavelength in about the range of 380-780 nm and that may be perceived by the normal unaided human eye'. In simple terms : Light is än electromagnetic radiation, that is visible to the human eye and is within a certain portion of the electromagnetic spectrum. Light is responsible for the sense of sight." Light travels fast and straight at a speed of about 186,000 miles per second [300,000 kilometers per second]. The straight paths of light are called LIGHT RAYS. the 3 ways to obstruct path of light are: To Block Light, Reflect Light, Bend it by changing its path from one medium to another.
Light is a form of energy which enables human beings and creatures to ‘see’ things. When light emitted from an object or reflected from the object enters our eyes we are able to see the object. We can’t see an object in the dark even if we are in light because there is no light coming from the object to our eyes.
Light is an electromagnetic radiation which exhibits properties like a wave as well as a particle. It always propagates in a straight line.
Light travels with a speed nearly equal to 3 [latex] \times[/latex][latex] 10^8 [/latex] m/s.
Optics: Optics is the branch of physics that studies electromagnetic radiation (for example, light and infrared radiation), its interactions with matter, and instruments used to gather information due to these interactions. Optics includes the study of sight. Thus, Optics essentially describes the behavior of visible, ultraviolet, and infrared light. Fact:Light is an electromagnetic wave, and hence other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties. Optics includes study of dispersion of light. Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.Light and Optics discusses some basics of the terms involved in the study of Light, an electromagnetic radiation.

shape Concepts

Light and Optics: Light is a form of energy which travels in straight lines and causes the sensation of vision. If we interpose a small obstacle between the object and our eyes, we are unable to see it.
Composition: In 1966, Isaac Newton passed a beam of light through a prism and found that it has seven colors, namely Violet, Indigo, Blue, Green, Yellow, Orange, Red. In these colors Violet will have the maximum energy.
  • The color of light depends on the nature of light falling on it and also on the constituent color of the incident light reflected by it.
  • Violet light has nearly twice the frequency of red light and half the wavelength.
  • In red light, green grass will appear black because it absorbs all colors except green and it would absorb the red rays of light falling on it.

Phenomenon of light:
Reflection: It is the bouncing back of light rays. The simple law of reflection is that the angle between the incoming ray and the perpendicular to the surface is equal to the angle between the reflected ray and the same perpendicular.
Refraction: It is the sudden change of direction of light when passing from one transparent substance into another. For example a ray of light passing from air into water bends #01a9ec">towards the perpendicular.
Diffraction: It is the spreading out of waves of light as it passes through a narrow aperture, because light is a wave of motion.
Dispersion: When white light is passed through glass prism, it is split into the colors of the spectrum. This is because each color has its own wavelength, which determines the angle at which light is refracted.
Scattering: A rough surface scatters light because each part of the surface reflects the light at light at different angles which is known as scattering of light. Absorption: Some surfaces absorb more of the light falling on them than others. If light of all colors is absorbed, the eyes see black. A black surface absorbs more light than a white surface.
Mirrors: Plane Mirrors work on the principle of reflection of light. When a man stands in front of a mirror, light from all parts of the body is reflected from the mirror back to the eyes and a virtual image appears to be formed behind the mirror.
Some Important Definitions: A Real Image is one through which the rays of light actually pass and which can be formed on a screen. A Virtual Image is one through which the rays, do not actually pass, although they appear to come from it.
Uses of Plane Mirrors: Plane mirrors find applications in optical level, sextant, kaleidoscope, periscope, telescope flat, dental mirror.
Uses of Concave mirrors: These are used in reflecting tele-scope, dental mirror, head lamp reflections, make-up mirrors.
Uses of Convex mirrors: Safety viewers at dangerous corners, anti-shop-lifting, car wing mirrors etc.
Curved Mirrors: There are two types of curved mirrors used for specific purposes viz., concave and convex. In Concave mirrors the light rays are reflected so as to converge to a point called the focus of the mirror. The virtual images produced by convex mirror are erect and smaller than the object. the convex mirror is therefore used as a rear-view mirror in vehicles as it has the advantage of a wide field of view.
Lenses: Lenses are used in all optical instruments, such as microscopes, telescopes, cameras, projectors. Lenses are of two types
Convex Lens: If a parallel beam of light is incident on a convex lens, all rays, after passing the lens, converge on a point called the principle focus. Convex lens surfaces have a real and therefore positive radius of curvature. It has a real and positive focal length.
Uses of Convex lenses: Convex lenses are used as magnifying glasses, eye, glasses to correct for long sight, microscope, telescope objective, camera and projectors.
Concave Lens: The rays of light spread out after passing through the lens. Concave lens surfaces have a virtual and therefore negative radius of curvature. It has real and positive focal length.
Uses of Concave lenses: Concave lenses are used as wide-angle spy hole in doors, glasses to correct for short sight, wide angle lens on coach rear window, eye lens in Galilean telescope.
Power of the Lens: The strength of a lens is described in terms of either its focal length or its power. The power of a lens is defined as Power of the Lens = [latex]\frac{1}{Focal\quad length\quad in\quad metres}[/latex] According to current theories, no material particle can travel at a speed greater than the speed of light.
Luminous and Non-luminous Objects: Luminous objects are those which emit their own light e.g., sun,glowworm, burning candle, electric lights. Non-luminous objects do not give out its own light but are visible only when light from a luminous object falls on it. e.g., moon, earth, table, paper, etc.
Transparent Translucent and Opaque materials: Transparent materials are those which allow most of light to pass through them. Example : Glass, water, air. Translucent materials allow only a part of light to pass through it. We cannot see distinctly through them. Example : greased paper, paraffin wax, etc. Opaque materials do not allow any light to pass through it. They reflect or absorb all the light that falls on them. Example : Books, desk, stone, rubber, trees, etc.
When light hits an opaque material, the light may be absorbed by the material and converted into heat energy. If the light is not absorbed, it is bounced back or reflected at the surface of the material. The turning back of light in the same medium is called the reflection of light.
Laws of reflection 1. The angle of incidence ‘i’ is equal to the angle of reflection ‘r’.
2. At the point of incidence, the incident rays, the normal to the surface and the reflected ray all lie in the same plane.
Plane Mirrors Plane mirror is a looking glass which is highly polished on one surface and is silvered on the other surface. When a light ray strikes the polished surface, it is reflected by the silvered surface. An ‘image’ is defined as the impression of an object carried over and formed by light reflected from it.
Use of plane mirrors (a) Plane mirrors are primarily used as looking glasses.
(b) Since, a combination of mirrors can produce multiple images, they are used to provide false dimensions in showrooms.
(c) They are also used as reflectors in solar cookers.
(d) Plane mirrors are used in the construction of a periscope.
Images and their properties An ‘image’ is defined as the impression of an object carried over and formed by light reflected from it. An image is said to be a real image if it can be caught on a screen, and a virtual image if it cannot be caught on the screen. For example, the image on the screen in a theater is a real image and the image observed in a plane mirror is a virtual image.
Real image 1. When the rays of light actually meet, the image so formed is known as real image.
2. A real image can be caught on a screen since it is formed by actual meeting of rays.
3. A real image is always inverted.
4. A real image is formed by a convergent reflected beam.
5. In ray diagrams, for real image, the rays are represented by full lines.
Virtual image 1. When the rays of light appear to meet, the image so formed is known as virtual image.
2. A virtual image cannot be caught on a screen since it is formed by meeting of imaginary rays.
3. A virtual image is always erect.
4. A virtual image is formed by a divergent reflected beam.
5. In ray diagrams, for virtual image, the rays are generally represented by dotted lines.
Characteristics of images formed by a plane mirror The image formed by a plane mirror is
(a) virtual (the image cannot be formed on a screen)
(b) upright
(c) laterally inverted (the left side of an image is formed by the right side of an object)
(d) the same size as the object
(e) the same distance behind the mirror as the object is in front of the mirror
Concave mirror : If the reflection takes place from the inner surface of a spherical mirror, then the mirror is called concave mirror.
Uses of concave mirrors : (i) In torches, search-lights and vehicles headlights to get powerful beams of light.
(ii) As a shaving mirror to see a large image of the face.
(iii) As a dentists mirror to see large images of the teeth of patients.
(iv) Large sized concave mirror is used to concentrate sunlight to produce heat in solar furnaces.
Convex mirror : If the outer surface of the spherical mirror acts as a reflector then the mirror is called convex mirror.
Uses of convex mirrors : (i) As a rear -view mirrors in vehicles.
(ii) For security purposes.
Mirror Formula If an object is placed at a distance u from the pole of a mirror and its image is formed at a distance v (from the pole) then, [latex]\frac{1}{v}[/latex]+[latex]\frac{1}{u}[/latex]=[latex]\frac{1}{f}[/latex]
Magnification If a thin object linear size O situated vertically on the axis of a mirror at a distance u from the pole and its image of size I is formed at a distance v (from the pole), magnification (transverse) is defined as
When a ray of light passes from one medium to another mediumit bends – towards the normal when goes from rarer to denser and away from the normal when goes from denser to rarer medium. This phenomenon is called refraction of light. Twinkling of stars, sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after actual sunset etc. due to atmospheric refraction.
Refractive index Refractive index of medium II with respect to medium I [latex]\mu_{21}[/latex] = [latex]\frac{Speed of light in medium I}{Speed of light in medium II}[/latex]
Laws of Refraction (i) Snell’s law : For any two media and for light of a given wavelength, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. i.e, [latex]\frac{sin i}{sin r}[/latex] = constant where i = incidence angle, r = refraction angle.
(ii)The incident ray, the refracted ray and the normal at the incident point all lie in the same plane
When object is in denser medium and observer is in rarer medium: Refractive Index[latex]\mu[/latex]= [latex]\frac{Real depth}{Virtual depth}[/latex]
A lens is a piece of transparent material with two refracting surfaces such that atleast one is curved and refractive index of used material is different from that of the surroundings.
Refraction through a thin lens (lens formula) If an object is placed at a distance u from the optical center of a lens and its images is formed at a distance v (from the optical centre) and focal length of this length is f then [latex]\frac{1}{v}[/latex]-[latex]\frac{1}{u}[/latex]=[latex]\frac{1}{f}[/latex] This is called lens formula.
Power of a lens The power of a lens is defined as P=[latex]\frac{1}{f (in m)}[/latex] The unit of power is diopter.
Focal length of a lens (lens maker’s formula) R1 = radius of curvature of first surface of lens, R2 = radius of curvature of second surface of lens.
When the object is placed in an optically denser medium and if the incident angle is greater than the critical angle then the ray of light gets reflected back to the originating medium. This phenomenon is called total internal reflection. Critical angle (ic): When a ray passes from an optically denser medium to an optically rarer medium, the angle of refraction r is greater than the corresponding angle of incidence i. From Snell’s law. Critical angle [latex](i_c)[/latex] Let [latex]\mu_1[/latex]= [latex]\mu[/latex], [latex]\mu_2[/latex]= 1 and let i=[latex](i_c)[/latex], r=[latex](90^0)[/latex] then [latex]sin[/latex] [latex](i_c)[/latex] = [latex]\frac{1}{\mu}[/latex] there fore [latex](i_c)[/latex]= [latex]sin^{-1}[/latex][latex]\frac{1}{\mu}[/latex] [latex](i_c)[/latex] is called the critical angle. This phenomenon takes place in shining of air bubble, sparkling of diamond, mirage, looming, in optical communication, endoscopy using optical fibre.
When a white ray of light or sunlight passes through a prism it breakes into its seven constituents colours violet, indigo, blue, green, yellow, orange and red (VIBGYOR). This phenomenon is called dispersion of light. The band of seven constituents colours is called spectrum. The deviation is maximum for violet colour and least for red colour.
The Rainbow A rainbow is a spectrum of white light from the sun. This is a phenomenon due to combined effect of dispersion, refraction and reflection of sunlight by spherical water droplets of rain.
(i) Primary rainbow: It is formed due to two refractions and one total internal reflection of the light incident on the droplet. Sunlight is first refracted as it enters a raindrop which cause different colours of light to separate. The observer sees a rainbow with red colour on the top and violet on the bottom.
(ii) Secondary rainbow: It is formed due to two refractions and two total internal reflection of light incident on the water droplet. It is due to four - step process. The intensity of light is reduced at the second reflection and hence the secondary rainbow is fainter than the primary rainbow. Scattering of Light As sunlight travels through the earth’s atmosphere it gets scattered by the small particles present in the atmosphere. According to Rayleigh law, the amount of scattering is inversely proportional to the fourth power of the wavelength ([latex]λ^4[/latex])
Phenomenon based on scattering of light (i) Blue colour of sky: Blue colour has a shorter wavelength than red colour therefore blue colour is scattered strongly. Hence the bluish colour predominates in a clear sky.
(ii) White colour of clouds: Clouds contain large dust particles, water droplets or ice particles. These large sized Particles do not obey Rayleigh law of scattering. All wavelengths are scattered nearly equally. Hence clouds are generally white.
(iii) Sun looks reddish at the Sunset or Sunrise: At sunset or sunrise, the sun’s rays have to pass through a larger distance in the atmosphere. Most of the blue and other shorter wavelengths are scattered. The least scattered light reaching our eyes, therefore the sun looks reddish.
Power of Accomodation of Eye: The ability of the lens to change its shape to focus near and distant objects is called accommodation. A normal human eye can see objects clearly that are between 25 cm and infinity Defects of Vision and Their Correction
Nearsightedness: If the eyeball is too long or the lens too spherical, the image of distant objects is brought to a focus in front of the retina and is out of focus again before the light strikes the retina. Nearby objects can be seen more easily. Eyeglasses with concave lenses correct this problem by diverging the light rays before they enter the eye. Nearsightedness is called myopia.
Farsightedness: If the eyeball is too short or the lens too flat or inflexible, the light rays entering the eye — particularly those from nearby objects— will not be brought to a focus by the time they strike the retina. Eyeglasses with convex lenses can correct the problem. Farsightedness is called hypermetropia.
Astigmatism: Astigmatism is the most common refractive problem responsible for blurry vision. Most of the eyeball’s focusing power occurs along the front surface of the eye, involving the tear film and cornea (the clear ‘window’ along the front of the eyeball). The ideal cornea has a perfectly round surface. Anything other than perfectly round contributes to abnormal corneal curvature– this is astigmatism. Cylindrical lens is use to correct astigmatism.
It is an optical instrument used to see magnified image of a tiny objects.
Resolving power (R.P.) of a Microscope: Resolving power (R.P.) of a microscope Resolving power of a microscope is defined as the reciprocal of the least separation between two close objects, so that they appear just separated, when seen through the microscope. Resolving power of a microscope [latex]\frac{1}{d}[/latex]= [latex]\frac{2μsinθ}{λ}[/latex] [latex]θ[/latex]= half angle of the cone of light from the point object [latex]μsinθ[/latex]= numerical aperture Telescope (Astronomical) It is an optical instrument used to increase the visual angle of distant large objects. It is used to see far off objects clearly.
Resolving power (R.P.) of a telescope: Resolving power of telescope is defined as the reciprocal of the smallest angular separation between two distant objects, so that they appear just separated, when seen through the telescope. Resolving power of telescope =[latex]\frac{D}{1.22λ}[/latex]
Interference of Light Waves: The phenomenon of redistribution of light energy in a medium due to superposition of light waves from two coherent sources is called interference of light.
Conditions for sustained interference: (i) Two sources must be coherent.
(ii) Amplitudes of waves should be either equal or approximately equal.
(iii) Light should be monochromatic.
Polarisation: It is the phenomenon of restricting the vibration of light in a particular plane. Light waves are transverse in nature i.e., the electric field vector associated with light wave is always at right angles to the direction of propagation of the wave. When unpolarised light is incident on a polaroid (Nicol Prism), the light wave gets linearly polarised i.e., the vibration of electric field vector are along a single direction.

shape Quiz 2

1. In which year Isaac Newton passed a beam of light through a prism and found that it has seven colors?
    A. 1966 B. 1967 C. 1968 D. 1970

Answer: Option A
2. What is the range of wavelength in electromagnetic radiation?
    A. 380-780 nm B. 780-880 nm C. 280-780 nm D. 480-880 nm

Answer: Option A
3. Red light is used in traffic signals, because?
    A. Colour of blood is red B. Animals can identify red C. Red light disperses least D. Red is the symbol of danger

Answer: Option C
4. An electric bulb has a filament made of?
    A. Copper B. Mercury C. Lead D. Tungstun

Answer: Option D
5. What is the proof of the fact that light travels in the same line?
    A. Sensation of vision B. Obstacles between the obstacles and eyes C. Formation of shadows D. The way it reflects

Answer: Option C
7. Who found out that beam of light when passed through prism consist seven colors?
    A. Issac Newton B. George Milton C. Alexander fleming D. Alfred Tennyson

Answer: Option A

shape Quiz

Q1. Which colour of light shows maximum deviation when passed through a prism ?
    A. White B. Red C. Violet D. Green Answer: C

Q2. Find the power of a convex lens if the image formed is at a distance of 16 cm from the lens when the object is placed on the other side of the lens at 20 cm from the optical centre?
    A. -3.75 diopters B. -11.25 diopters C. 3.75 diopters D. 11.25 diopters Answer: D

Q3. Light travels fastest in
    A. Nitrogen B. Air C. Steel D. Vacuum Answer: A

Q4. Which is the light sensitive cell present on retina and sensitive to intensity of light :
    A. Rods B. Cones C. Both rods and cones D. None of these Answer: A

Q5. What is used as a reector in the headlights of a vehicle?
    A. Concave mirror B. Concave lens C. Convex lens D. Convex mirror Answer: A

Q6. What will be the focal length of the spherical mirror of radius of curvature of 50 cm?
    A. 25 cm B. 100 cm C. 10 cm D. 50 cm Answer: A

Q7. Which of the these travels in glass with minimum velocity?
    A. Green light B. Red light C. Violet light D. Yellow light Answer: C

Q8. Which lenses used to remove the defect of presbyopia?
    A. Concave lens B. Convex lens C. Bifocal lens D. Both A and B Answer: C

Q9. When the position of object in convex lens is at infinity, the size of image will be
    A. Highly diminished B. Of same size C. Highly enlarged D. Little diminished Answer: A
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