Example 1
A ray hits a mirror at 35∘ to the normal. Find its reflection angle to the normal.
The reflection angle equals the incidence angle, so it is 35∘. The angle to the mirror surface would be 90−35=55∘.
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Year 9
Light, reflection and refraction
Apply wave rules to reflection, changing media and the electromagnetic spectrum.
Method and worked examples
Light is electromagnetic radiation and can cross a vacuum. The electromagnetic spectrum includes radio, microwave, infrared, visible, ultraviolet, X-rays and gamma rays. These have different frequencies and wavelengths but share the same vacuum speed: about 300 million metres per second. A simple wave model represents light as transverse.
At a smooth mirror, incidence angle equals reflection angle, both measured from the normal perpendicular to the surface. Refraction occurs when light changes speed between media. Entering glass from air at an angle, light usually bends towards the normal; leaving glass for air it bends away. Frequency stays unchanged across the boundary while speed and wavelength change.
Example 1
Example 2
Light enters glass from air at an angle and slows down. What happens to direction and frequency?
It bends towards the normal. Its frequency stays the same while its wavelength becomes shorter.
Watch out: Both incidence and reflection angles are measured from the normal, perpendicular to the surface.