Physics Study Notes on LIGHT



REFLECTION OF LIGHT (Law of Reflection)
(i) The angle of incidence is equal to the angle of reflection.
(ii) The incident ray, the normal, the point of incidence and the reflected ray, all lie in the same plane.
Spherical Mirrors & their Uses

Uses of concave mirrors

  • Concave mirrors are commonly used in torches, search-lights and vehicles headlights to get powerful parallel beams of light.
  • They are often used as shaving mirrors to see a larger image of the face. The dentists use concave mirrors to see large images of the teeth of patients.
  • Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.


Uses of convex mirrors
Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles, enabling the driver to see traffic behind him/her to facilitate safe driving. They always give an erect, though diminished, image. Also, they have a wider field of view as they are curved outwards. Thus, convex mirrors enable the driver to view a much larger area than would be possible with a plane mirror.

REFRACTION OF LIGHT 
The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. When a thick glass slab is placed over some printed matter, the letters appear raised when viewed through the glass slab the bottom of a tank or a pond containing water appears to be raised seen a pencil partly immersed in water in a glass tumbler. It appears to be displayed at the interface of air and water.
A lemon kept in water in a glass tumbler appears to be bigger than its actual size when viewed from the sides.

The following are the laws of refraction of light :
(i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
(ii)The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for the light of a given color and for the given pair of media. This law is also known as Snell’s law of refraction. If i is the angle of incidence and r is the angle of refraction, then,
Sin i/Sin r = constant.
The one with the larger refractive index is optically denser medium than the other. The other medium of the lower refractive index is optically rarer. The speed of light is higher in a rarer medium than a denser medium.

DISPERSION OF WHITE LIGHT BY A GLASS PRISM
The prism has probably split the incident white light into a band of colors. The sequence of colors VIBGYOR. The splitting of light into its component colors is called dispersion.
Different colors of light bend through different angles with respect to the incident ray, as they pass through a prism. The red light bends the least while the violet the most. Thus the rays of each color emerge along different paths and thus become distinct. It is the band of distinct colors that we see in a spectrum.
A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colors reach the observer’s eye.

ATMOSPHERIC REFRACTION
The air just above the fire becomes hotter than the air further up. The hotter air is lighter (less dense) than the cooler air above it and has a refractive index slightly less than that of the cooler air. Since the physical conditions of the refracting medium (air) are not stationary, the apparent position of the object, as seen through the hot air, fluctuate. This wavering is thus an effect of atmospheric refraction (refraction of light by the earth’s atmosphere).

Twinkling of stars
The twinkling of a star is due to atmospheric refraction of starlight.

Advance sunrise and delayed sunset
The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction. 

SCATTERING OF LIGHT
Scattering of light is the phenomenon by which a beam of light is redirected in many different directions when it interacts with a particle of matter.
The blue color of the sky, the color of the water in the deep sea, the reddening of the sun at sunrise and the sunset.

Total Internal Reflection 
Total internal reflection is a phenomenon that occurs when light travels from a more optically dense medium to a less optically dense one, such as glass to air or water to air.
Examples of Total Internal Reflection -
(a) Mirage – Hotter air is less dense, and has a smaller refractive index than the cooler air. On hot summer days, the air near the ground becomes hotter than the air at higher levels noticed that while moving in a bus or a car during a hot summer day, a distant patch of road, especially on a highway, appears to be wet. This is also due to a mirage.
(b) Diamonds - Their brilliance is mainly due to the total internal reflection of light inside them.
(c) Optical fibers too make use of the phenomenon of total internal reflection. Light undergoes repeated total internal reflections along the length of the fiber there is no appreciable loss in the intensity of the light signal. 

Tyndall Effect
The Tyndall effect is the scattering of light as a light beam passes through a colloid. The individual suspension particles scatter and reflect light, making the beam visible.
The earth’s atmosphere is a heterogeneous mixture of minute particles like smoke, tiny water droplets, suspended particles of dust and molecules of air. When a beam of light strikes such fine particles, the path of the beam becomes visible. 

Tyndall effect is seen when a fine beam of sunlight enters a smoke-filled room through a small hole. Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest.

Power of Accommodation 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. The cylindrical lens is used to correct astigmatism.

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