Waves and boundaries for OCR GCSE Physics
This page covers the following topics:
3. Absorption and transmission
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.
When waves arrive at the boundary between two materials, they can be reflected, transmitted or absorbed. What happens to the wave energy depends on the following properties: the type of wave, the wavelength of the wave and the difference between the two media. The shorter the wavelength of the wave, the easier it is for it to be trasmitted.
State whether the following situation is an example of absorption or transmission of wave energy: the waves of the sea hitting the beach.
When the waves of the sea hit the beach, they are stopped by the sand, as most of the wave energy is taken by the sand. Therefore, this is an example of absorption.
Refraction is the change in direction of a ray at a boundary between two transparent materials due to their different densities.
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˚.
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˚.
Explain why pavements get hot during the summer.
During the summer, when light waves from the Sun hit the pavement, absorption occurs, meaning that the wave loses most of its wave energy to the pavement. Thus, the pavement gains energy which is transformed into heat energy, and so heats up.
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