Lenses

Bending of light when it travels from one medium to another

It is refracted towards the normal and vice versa

• The inciedent ray, the normal and the refracted ray all lie on the same place

• the ratio of the sin of the angle of incidence to the sin of the angle of refraction is a constant called the refractive index

For 2 given media, the ratio ( sin i / sin r ) is a constant, where is the angle of incidence and r the angle of reflaction

When the angle of incidence reaches a certain (critical) angle, the light no longer refracts outward, but instead travels along the surface

• the critical angle corresponds to the angle of incidence in the dense medium which produces an angle of 90° in the rare medium

The critical angle corresponds to the angle of incidence in the dense medium which produces an angle of refraction of 90° in the rare medium

Occurs when the angle of incidence is greater than the critical angle and light is reflected back into the denser medium

equipment: rectangular glass block, sheet of paper and drawing board, four pins, ruler, protactor

Method:

• first you place the paper on the drawing board and then place the glass block on the paper

• Mark the outline of the glassblock on the paper and dont move it afterwards

• Stick two pins A,B on the paper, so that it makes an acute angle with one face of the block

• Now u look on the other side of the block, find the angle when pin b completely hides pin a and mark this with pin c

• Mark the pins with a circle around them and remove the pins

• Draw the ray of incident and the point of emergent ray

  Draw a straight line joining the point of incidence to the point of emergence - this is the refracted ray inbetween the block

• Draw the norm with the protector and measure the angle of incidence i and the angle of refraction r

• Repeat this with different values of the angle of incidence