Law of Reflection

(Triple Only)

When waves reach the boundary between two different materials, they can be reflected, transmitted, or absorbed.

• Reflected: the wave bounces back from the boundary/surface into the original medium.

• Transmitted: the wave passes through the boundary into the new material (often changing speed and direction).

• Absorbed: the wave’s energy is transferred to the material, usually becoming thermal (heat) energy.

Reflection is how we are able to see our image in a mirror. When a ray of light hits the surface of a mirror, it is reflected back into our eyes.

When drawing reflection diagrams, mirrors are represented using a straight line with shading on one side. The shading shows the non-reflective side, so it is clear which side of the mirror reflects the light. These are known as plane mirrors.

When drawing a reflection diagram, the incoming light ray is called the incident ray, and the light ray that rebounds is called the reflected ray. We also need to draw an imaginary, reference line at to the surface - this is called the normal. The normal is very important, as we measure the incident angle and the reflected angle against this line - not the mirror.

Law of Reflection

The law of reflection states that the incident angle equals the reflected angle, . For example, in the below diagram we can see that the incident angle is , so the reflected angle must also be . Always use a protractor when drawing and measuring these angles!

Specular and Diffuse Reflection

There are two types of reflection, specular and diffuse.

In specular reflection, incident rays of light that are parallel to each other are reflected parallel to each other, allowing a clear image to be seen. This occurs in mirrors, where the surfaces are smooth, and on calm water, where the lake or water surface is smooth, allowing you to see an image of yourself.

In diffuse reflection, the surface is uneven, resulting in incident rays of light that are all parallel to each other being reflected at different angles - the reflected light is scattered. The image is not clear, like when looking at an image of yourself in a lake when the surface is disturbed, such as if your friend jumps in, creating ripples across the lake.

Reflection Required Practical

• Place a plane mirror upright on a piece of paper and mark its position with a pencil

• Use a ray box to shine a narrow beam of light at the mirror

• Mark the incident and reflected light rays on the paper with crosses, so that when the light is turned off, you can connect the crosses to mark its path.

• At the point where the ray meets the mirror, draw a normal at right angles to the mirror surface

• Use a protractor to measure the angle of incidence and the angle of reflection

• Repeat the steps above for different angles of incidence

The results show that the angle of incidence always equals the angle of reflection.

The experiment could be improved by checking the mirror surface for defects such as bumps or dents, as we need a smooth surface for specular reflection. Also, repeating the experiment for each angle of incidence at least three times and calculating the mean will improve the experiment's reliability.

We can adapt the experiment to see if different surfaces, such as polished metal or paper, also follow this rule. .

Light emitted from the Sun or from light bulbs is called white light. White light is made up of seven different colours (the colours of the rainbow): Red, Orange, Yellow, Green, Blue, Indigo and Violet. These colours can be remembered using the mnemonic -> Richard Of York Gave Battle In Vain (or ROY G BIV).

Each colour of light has a different wavelength and frequency. Red light has the longest wavelength and violet light the shortest. We know white light is made of these colours, as when white light passes through a glass prism, the different colours spread out and can be seen as they travel at different speeds through the glass. This process is called dispersion. More on this on the EM waves page.

• Transparent objects allow all incident light to pass through, and thus can be seen through clearly (e.g., clear glass).

• Translucent objects allow some of the incident light to pass through, but not all, which is why they appear blurred (e.g., frosted glass).

• Opaque objects allow no light to pass through them; either all incident light is absorbed by the material, causing it to heat up (like black objects), or all incident light is reflected away, and none is absorbed (like white objects). Coloured objects absorb all incident light except the colour that is reflected into our eyes (e.g., a red apple absorbs all colours of light except red light that it reflects into our eyes).

Coloured Filters

Coloured filters work by absorbing some colours of light and transmitting others. For example, A red filter transmits red light and absorbs all other colours. This means only red light passes through the filter. All objects viewed through this filter will appear shades of red, depending on whether they reflect the incident red light into our eyes or absorb it, appearing black to us (as no light is reflected into our eyes, as red light is the only colour incident on the object when viewed through a red filter).

The primary colours of light that our eyes can detect are red, green and blue, which can be combined to make the secondary colours as shown in the image below. When all colours are combined equally, they produce white light.