# Waves and boundaries for SQA National 5 Physics

1. Reflection
2. Refraction

Waves can be reflected at boundaries between two materials and obey the Law of Reflection. The Law of Reflection states that the angle of incidence of the wave should be equal to the angle of reflection of the wave. These angles are measured by taking the angle between the wave and the normal line, which is a line perpendicular to the surface of the boundary.

Refraction is the change in direction of waves that occurs at a boundary of the two transparent materials. Refraction occurs due to the fact that different materials have different densities. A ray bends towards the normal when it is transmitted from a less dense through to a more dense material, since the denser material causes its speed to slow down. When the ray is passed from a more dense to a less dense material, it will bend away from the normal as it will speed up. Refraction appears as optical illusions to the human eye, as it cannot understand the concept of light changing direction.

# 1

Define refraction.

Refraction is the change in direction of a ray at a boundary between two transparent materials due to their different densities.

# 2

Given that the sum of the angle of incidence and the angle of reflection is 96˚, find the angle of incidence.

Let x = angle of incidence. By the Law of Reflection, angle of incidience = angle of reflection. Therefore, x = angle of incidence = angle of reflection. So, x + x = 96, thus 2x = 96. Therefore x = 48˚.

# 3

A light ray is reflected off a plane mirror at an angle of 52˚. What is the angle of incidence of the light ray?

By the Law of Reflection, the angle of incidence = angle of reflection. So, angle of incidence = 52˚.

# 4

Explain what happens to a light ray moving through air when it is transmitted through glass.

Glass is denser than air, therefore when the light ray is transmitted through glass, it will slow down. Therefore, refraction will occur as the light ray will change direction by bending towards the normal.

# 5

A light ray is transmitted from water through to air. Given that the two angles are 28˚ and 34˚, explain which is the angle of incidence and which is the angle of refraction.

Water is denser than air, so when the light ray moves from water through to air, it speeds up and bends away from the normal due to refraction. Since it bends away from the normal, the angle of refraction must be greater than the angle of incidence. Therefore, the angle of incidence = 28˚ and the angle of refraction = 34˚.

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