Features of Light Revision Notes

In these revision notes for Features of Light, we cover the following key points:

• What are rays? What are beams? Where do they differ from each other?
• How does light propagate?
• Why light is an EM wave?
• What are the main components of an EM wave?
• Which are the colours of visible spectrum of light? How are they determined?
• How many types of materials are there concerning their ability to allow light pass through them?
• What is shadow? Which are the conditions for the shadow to produce?
• What are the sub-categories of shadow? What are their features?

Features of Light Revision Notes

We often use terms like "ray" and "beam" to represent light and its propagation is space. Ray and beam are artificial constructs.

Ray is usually associated with geometric optics and helps us understand the light propagation. A ray is one-dimensional and has a zero cross-sectional area (zero thickness), and therefore it is not physical.

Beams have a finite cross-section and they are typically associated with lasers, for example. Hence, if we use the language of geometry we can compare rays with straight lines and beams with long cylinders of light. In this sense, you can think of the beam as being "made up of many rays" - so beams can be thought of as "bundles of rays".

Light rays and beams are a proof that light propagates in straight lines, despite their sinusoidal form.

Light, as a kind of EM wave carries energy, not matter. As such, it is produced by a source which when it is very hot becomes incandescent (emits light). Light is part of EM spectrum, i.e. unlike mechanical waves, it possesses both an electric and magnetic component (E and H) which are perpendicular to each other. These components are represented by the respective fields (electric and magnetic field). Thus, all EM waves contain two sinusoidal equations: one for each field. They are:

for electric field and

for magnetic field.

The electric field component is much greater than the magnetic one. The ratio between them gives the speed of light c.

The value 3 × 108 m/s for the speed of light in vacuum is a rounded value. Accurate measurements give the value c = 299 792 458 m/s.

Visible light is a part of EM spectrum. Our brain perceives different wavelengths of visible light as colours. The order of colours from the least powerful to the most powerful (from the less energetic to the most energetic) a human can see is:

Red → Orange → Yellow → Green → Cyan → Blue → Violet

Light cannot pass through all media. Some materials allow light pass through them. They are known as transparent materials or media. Vacuum, air, glass, water etc. are examples of transparent materials.

Any light beam penetrates regularly through transparent materials. As a result, we see everything through them.

The opposite of transparent is opaque, i.e. in such materials light cannot pass through. Metals, concrete, wood, etc. are examples of opaque materials.

There is also a third category of materials in which light passes only partially across. Furthermore, the beam is not regular anymore after passing through them. These materials are known as translucent.

Not all light waves are able to pass through such materials because some of the light waves falling on a translucent material are absorbed by the material itself (light is transmitted only partially through them). Furthermore, they will diffuse after leaving the translucent medium.

Examples of translucent materials include thin plastic materials such as plastic bags, dirty water, clouds, blurry glass, etc.

Since opaque objects do not allow light rays to pass through them, the space behind such objects will be less illuminated. As a result, a dark region called shadow will exist in that part.

There are two types of shadows: umbra and penumbra. Umbra represents a complete shadow; it is formed when the light source is very small or when it is far away from the opaque objects (such as the Sun). In this case, an umbra is formed behind the object. Furthermore, the umbra borders are sharp.

When the light source is relatively large and it cannot be considered as a point light source, or when there are two or more point sources emitting light, there will be regions behind an opaque object, which are partially illuminated. As a result, a dim shadow (half-shadow or penumbra) is formed on the ground in these regions.