Refraction occurs when light travels from one transparent medium to another at a different speed.
The following are the laws of refraction of light.
- The incident ray, the refracted ray and the normal ray to the interface of two transparent media at the point of incidence all lie in the same plane.
- 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 colour and the given pair of media. This law is also known as Snell’s law of refraction. (This is true for angle \(0^°<i<90^°\))
If \(i\) is the angle of incidence and \(r\) is the angle of refraction, then
\(\frac{sin\ i}{sin\ r}\ =\ constant\)
Refraction through glass slab:
A medium's absolute refractive index is simply referred to as its refractive index.
The refractive index of water is \(n_w\ =\ 1.33\).
This means that the ratio of light speed in air to light speed in water is equal to \(1.33\).
Similarly, crown glass has a refractive index of \(n_g\ =\ 1.52\).
PYQ:
Exam tips:
1. Refractive index
2. Finding medium
3. Values to be notes
4. Formula
Lens:
A lens is formed by a transparent material bound by two surfaces, one or both of which are spherical. A lens has two spherical surfaces, one convex and the other concave. Each of these surfaces is a sphere in its own right.
Image formation by convex lens:
Image formation by concave lens:
The lens equation is given by,
\(\frac{1}{f}\ =\ \frac{1}{v}\ -\ \frac{1}{u}\)
The magnification is given by,
\(M\ =\ \frac{v}{u}\ =\ \frac{h'}{h_o}\)
Power of a lens:
The power of a lens is the degree of convergence or divergence of light rays it achieves.
\(P\ =\ \frac{1}{f}\)
The 'dioptre' is the SI unit of lens power.
A lens with a focal length of one metre has a power of one dioptre.
The power of a convex lens is positive, while the power of a concave lens is negative.
PYQ:
Exam tips:
Exam tip:
1. Type of lens
2. Sign concention
3. Unit conversion
4. Magnification