Properties Of Light Revision

Revision: Properties Of Light

When light interacts with objects or a medium, such as glass or water, it displays certain properties: it can either be reflected, absorbed or transmitted.

A landscape reflection from a still lake.

To describe the reflection of light, we will use the following terminology. The incoming light ray is called the incident ray. The light ray moving away from the surface is the reflected ray. The most important characteristic of these rays is their angles in relation to the reflecting surface. These angles are measured with respect to the normal of the surface. The normal is an imaginary line perpendicular to the surface. The angle of incidence, \(\theta_i\) is measured between the incident ray and the surface normal. The angle of reflection, \(\theta_r\) is measured between the reflected ray and the surface normal. This is shown in the figure below.

The angles of incidence and reflection are measured with respect to the normal to the surface.

When a ray of light is reflected, the reflected ray lies in the same plane as the incident ray and the normal. This plane is called the plane of incidence and is shown in the figure below.

The plane of incidence is the plane including the incident ray and the normal to the surface. The reflected ray also lies in the plane of incidence.

The simplest example of the law of reflection is if the angle of incidence is \(\text{0}\)\(\text{°}\). In this case, the angle of reflection is also \(\text{0}\)\(\text{°}\). You see this when you look straight into a mirror.

When a light ray strikes a surface at right angles to the surface, then the ray is reflected directly back.

Applying what we know from the law of reflection, if a light ray strikes a surface at \(\text{60}\)\(\text{°}\) to the normal to the surface, then the angle that the reflected ray makes with the normal must also be \(\text{60}\)\(\text{°}\) as shown in the figure below.

Ray diagram showing angle of incidence and angle of reflection. The law of reflection states that when a light ray reflects off a surface, the angle of reflection \(\theta_r\) is the same as the angle of incidence \(\theta_i\).

Real world applications of reflection

A parabolic reflector is a mirror or dish (e.g. a satellite dish) which has a parabolic shape. Some examples of very useful parabolic reflectors are car headlamps, spotlights, telescopes and satellite dishes. In the case of car headlights or spotlights, the outgoing light produced by the bulb is reflected by a parabolic mirror behind the bulb so that it leaves as a collimated beam (i.e. all the reflected rays are parallel). The reverse situation is true for a telescope where the incoming light from distant objects arrives as parallel rays and is focused by the parabolic mirror to a point, called the focus, where an image can be made. The surface of this sort of reflector has to be shaped very carefully so that the rays all arrive at the same focal point.

On the left is a ray diagram showing how a telescope mirror works to collect incoming incident light (parallel rays) from a distant object such as a star or galaxy and focus the rays to a point where a detector e.g. a camera, can make an image. The diagram on the right shows how the same kind of parabolic reflector can cause light coming from a car headlight or spotlight bulb to be collimated. In this case the reflected rays are parallel.

Absorption

In addition to being reflected, light can also be absorbed. Recall from grade 10 that visible light covers a range of wavelengths in the electromagnetic spectrum. The colours we see with our eyes correspond to light waves with different wavelengths or frequencies.

Imagine that you shine a torch or other source of white light onto a white piece of paper. The light is reflected off the paper and into our eyes and we see the colour of the paper as white. Now if we were to shine the same light onto a red apple we will notice that the colour of the apple appears red. This means that the surface of the apple is only reflecting red light into our eyes. All the other wavelengths in the incident white light are absorbed by the apple's skin. If you touch the apple, it will feel warm because it is absorbing the energy from all the light it is absorbing. Because of absorption and reflection, we can perceive colours of different objects. White objects reflect all or most of the wavelengths of light falling on them, coloured objects reflect particular wavelengths of light and absorb the rest. Black objects absorb all the light falling on them. This is why wearing a white t-shirt outside in the sun is cooler than wearing a black t-shirt, since the white t-shirt reflects mostof the light falling on it, while the black t-shirt will absorb it and heat up.

Transmission

A further property of light is that it can be transmitted through objects or a medium. Objects through which light can be transmitted are called transparent and objects which block out light or that light cannot pass through are called opaque.

For example, glass windows allow visible light to pass through them which is why we can see through windows. The light rays from things outside the window can pass through or be transmitted through the glass and into our eyes. Brick walls on the other hand are opaque to visible light. We cannot see through brick walls because the light cannot be transmitted through the wall into our eyes. The transmission of light through an object depends on the wavelength of the light. For example, short wavelength visible light cannot be transmitted through a brick wall whereas long wavelength radio waves can easily pass through walls and be received by a radio or cell phone. In other words, a brick wall is transparent to radio waves!