Light – Reflection and Refraction

Light is a form of energy that enables us to see the world around us. It travels in a straight line and can undergo different phenomena when it interacts with matter, such as reflection, refraction, dispersion, scattering, and absorption. Among these, reflection and refraction are the most fundamental, and they form the basis for many applications in daily life. This chapter focuses on how light behaves when it strikes mirrors and lenses, and how we can use mathematical relationships to predict the properties of images formed.

Reflection of Light

Reflection is the phenomenon in which light bounces back after striking a polished surface such as a mirror. There are two types of reflection – regular reflection from smooth surfaces, and diffuse reflection from rough surfaces. Regular reflection helps in the formation of clear images, while diffuse reflection is responsible for the visibility of objects from all directions.

The laws of reflection are:

• The angle of incidence is equal to the angle of reflection.
• The incident ray, reflected ray, and normal to the reflecting surface lie in the same plane.

These laws are valid for both plane and spherical mirrors.

Spherical Mirrors

A spherical mirror is a part of a hollow sphere whose inner or outer surface is polished.

• Concave mirror: Inner surface reflecting, converges light.
• Convex mirror: Outer surface reflecting, diverges light.

Key terms:

• Pole (P): The centre of the mirror surface.
• Centre of curvature (C): Centre of the original sphere.
• Principal axis: The line joining P and C.
• Focus (F): Point where rays parallel to principal axis meet or appear to diverge.
• Focal length (f): Distance PF.

Image formation by concave mirrors depends on the object’s position. For example, an object beyond C forms a real, inverted, and diminished image between C and F. A convex mirror always forms a virtual, erect, and diminished image.

The relationship between distances is given by the mirror formula:
[
\frac{1}{f} = \frac{1}{v} + \frac{1}{u}
]
where f = focal length, u = object distance, v = image distance.
Magnification is expressed as:
[
M = \frac{h_i}{h_o} = \frac{v}{u}
]

Applications: Concave mirrors are used in shaving mirrors, headlights, solar cookers, while convex mirrors are used as rear-view mirrors in vehicles.

Refraction of Light

Refraction is the bending of light when it passes from one medium to another, caused by a change in speed. When light travels from a rarer medium (like air) to a denser medium (like glass), it bends towards the normal. When it travels from a denser to a rarer medium, it bends away from the normal.

Laws of refraction:

1. Incident ray, refracted ray, and normal lie in the same plane.

2. Snell’s law:
[
\frac{\sin i}{\sin r} = \mu
]
where i = angle of incidence, r = angle of refraction, μ = refractive index.

The refractive index of a medium is also defined as the ratio of the speed of light in vacuum to the speed of light in the medium.

Refraction through a Glass Slab

When light enters a rectangular glass slab, it bends towards the normal at the first surface and away from the normal at the second surface. The emergent ray is parallel to the incident ray but laterally displaced. This proves that refraction changes the path but not the direction of propagation if the surfaces are parallel.

Refraction by Spherical Lenses

A lens is a transparent material bounded by spherical surfaces.

• Convex lens (converging): Thicker at the middle.
• Concave lens (diverging): Thinner at the middle.

Key terms:

• Optical centre (O): Centre point of the lens.
• Principal axis: Line passing through optical centre and centres of curvature.
• Principal focus (F): Point where rays parallel to the axis converge (convex) or appear to diverge (concave).

The lens formula is:
[
\frac{1}{f} = \frac{1}{v} - \frac{1}{u}
]
Magnification is given as:
[
M = \frac{h_i}{h_o} = \frac{v}{u}
]
The power of a lens (in dioptre, D) is defined as:
[
P = \frac{100}{f(cm)}
]

Applications of lenses include microscopes, telescopes, cameras, projectors, and spectacles.

Practical Uses of Reflection and Refraction

• Concave mirrors: Solar cookers, headlights, dentist’s mirror.
• Convex mirrors: Used in vehicles for wider field of view.
• Convex lenses: Used in magnifying glasses, spectacles, optical instruments.
• Concave lenses: Correcting myopia (short-sightedness).

Conclusion

Reflection and refraction are basic phenomena of light that explain how images are formed in mirrors and lenses. By applying the mirror and lens formulae, we can predict the size, orientation, and nature of images. These concepts are not only important for understanding natural phenomena but also for designing devices like cameras, spectacles, microscopes, and telescopes. Mastering this chapter gives a strong foundation for further studies in optics and real-life applications of physics.