Refraction through Prism — Definition
Definition
Imagine you have a piece of glass shaped like a triangle, specifically a triangular prism. When a beam of light, say from a laser pointer, hits one flat side of this glass and then exits from another flat side, something interesting happens: the light changes direction not once, but twice.
This bending of light as it passes from one medium (like air) into another (like glass) and then back into the first medium (air) is called refraction. A prism is designed to demonstrate this effect clearly.
\n\nWhen light enters the prism from the air, it slows down and bends towards the normal (an imaginary line perpendicular to the surface). Then, as it travels through the glass and hits the second surface to exit back into the air, it speeds up and bends away from the normal.
Because the two surfaces of the prism are inclined to each other, these two refractions don't cancel each other out. Instead, they combine to make the light ray deviate, or bend, significantly from its original path.
Crucially, the light always bends towards the base of the prism. \n\nThe amount by which the light bends is called the 'angle of deviation'. This angle depends on several factors: the angle at which the light first hits the prism (called the angle of incidence), the angle between the two refracting surfaces of the prism (called the angle of the prism or refracting angle), and how much the prism material slows down light (its refractive index).
\n\nThere's a special condition called 'minimum deviation'. If you slowly change the angle at which the light enters the prism, you'll notice that the angle of deviation first decreases, reaches a minimum value, and then starts increasing.
This lowest possible deviation is the minimum deviation. It occurs when the light ray travels symmetrically through the prism, meaning the angle of incidence on the first surface is equal to the angle of emergence from the second surface.
Understanding this phenomenon is key to explaining how prisms can split white light into its constituent colors, a process known as dispersion, which is responsible for rainbows.