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Nature of Light

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NATURE OF LIGHT

Our investigation of light revolves around two questions of fundamental importance:

What is the nature of light?
How does light behave under various circumstances?

Answers to these questions can be found in Maxwell’s equations (in Electromagnetic Waves), which predict the existence of electromagnetic waves and their behavior. Examples of light include radio and infrared waves, visible light, ultraviolet radiation, and X-rays. Interestingly, not all light phenomena can be explained by Maxwell’s theory. Experiments performed early in the twentieth century showed that light has corpuscular, or particle-like, properties. The idea that light can display wave and particle characteristics is called wave-particle duality, examined in Photons and Matter Waves.

Law of Reflection

Whenever we look into a mirror, or squint at the sunlight glinting from a lake, we are seeing a reflection.

When you look at a piece of white paper, you see light scattered from it. Large telescopes use reflection to form an image of stars and other astronomical objects

The law of reflection states that: The angle of reflection equals the angle of incidence.

Exercise

For the figure given below, select the relation between the $θ_{i}$ and $θ_{r}$ .

$θ_{i} . θ_{r} = 90^{ο}$

$θ_{i} + θ_{r} = 180^{o}$

$θ_{i} = θ_{r}$

$θ_{r} = - θ_{i}$

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We expect to see reflections from smooth surfaces, but the figure below illustrates how a rough surface reflects light.

Since the light strikes different parts of the surface at different angles, it is reflected in many different directions, or diffused. Diffused light is what allows us to see a sheet of paper from any angle, as shown in Figure (a). People, clothing, leaves, and walls all have rough surfaces and can be seen from all sides. A mirror, on the other hand, has a smooth surface (compared with the wavelength of light) and reflects light at specific angles, as illustrated in Figure (b). A combination of these effects takes place when the Moon reflects from a lake, as shown in Figure (c).

Exercise

A light ray strikes a reflective plane surface at an angle of $37^{\circ}$ with the surface.

37°

Calculate the angle of incidence.

$θ_{i} =$ $^{\circ}$

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Refraction

The changing of a light ray’s direction (loosely called bending) when it passes through substances of different refractive indices is called refraction and is related to changes in the speed of light, $v = \frac{c}{n}$ .

Refraction is responsible for a tremendous range of optical phenomena, from the action of lenses to data transmission through optical fibers.

The below figure how you may see the same fish appearing to be in two different places, this is because of refraction only.

The figure below shows how a ray of light changes direction when it passes from one medium to another. As before, the angles are measured relative to a perpendicular to the surface at the point where the light ray crosses it.

The change in direction of the light ray depends on the relative values of the indices of refraction of the two media involved. In the situations shown, medium two has a greater index of refraction than medium.

The change in direction of a light ray as it passes from one medium to another depends on both the incident angle and the change in speed (refractive index) of the light wave. This relationship is described by Snell's law, which governs the behavior of light at the boundary between two different transparent media.

$n_{1} {sinθ}_{1} = n_{2} {sinθ}_{2}$

Here $n_{1}$ and $n_{2}$ are the indices of refraction for media $1$ and $2$ , and $θ_{1}$ and $θ_{2}$ are the angles between the rays and the perpendicular in media $1$ and $2$ .

Exercise

Find the index of refraction for the medium $1$ in Figure(a) above, assuming the medium $1$ is air and given that the incident angle and the refraction angle$32^{\circ}$ and the angle of refraction is $21^{\circ}$.

$n_{2} =$ (Upto two decimal places only)

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