### KS3 Physics

Motion and Forces
Waves
Matter

# Light Waves

Light waves travel in straight lines as transverse waves. The vibrations in transverse light waves occur at a perpendicular (90°) angle to the direction in which the light travels.

Visible light is the light we detect with our eyes. As you can see in the diagram below, light reflects off an object and into our eyes.

• To see the leaves, light reflects off them and enters our eyes.

Light can travel through space, which is a vacuum, because it doesn’t need particles to travel, unlike sound.

## Objects Transmitting Light

Light can also travel through gases, liquids and some solids. When light can pass through an object, we say the object can transmit light.

Transparent, translucent and opaque are terms we use to describe how objects transmit light. Objects that transmit light (which means to let light through) are either transparent or translucent and objects that can not transmit light are called opaque objects.

### Transparent objects

Transparent is the word used to describe something that is clear or see through. If an object is transparent, you can see a clear image through the object. An example of this is a glass window.

As glass is transparent (see through), it transmits light.

Most of the light hitting the glass passes through, allowing you to see a clear image. However, some light is also reflected, which is why you can often see a clear reflection in the glass.

### Translucent objects

Translucent is a word used to describe something that transmits light but does not allow you to see a clear image through it. For example, frosted glass is a translucent object.

Some of the light that hits the glass passes through but the light that is transmitted gets scattered, which is why you can only see fuzzy images through frosted glass.

The remaining light that hits the glass is reflected, and this reflected light is also scattered, making the reflection unclear as well.

### Opaque objects

The word opaque is used to describe an object in which light is not capable of passing through.

Opaque objects are those that do not transmit light. Most of the light that hits opaque objects is absorbed, however, some light is reflected.

Any object you can’t see through is considered opaque.

## Speed of Light

The speed of light is approximately 300,000,000 m/s, which is much faster than the speed of sound. It takes just over 8 minutes for light to travel from the Sun to Earth.

This explains why you see lightning before you hear it. Although the light and sound are produced at the same time, the light reaches your eyes before you are able to hear the sound.

## Luminous and Non-Luminous Objects

Objects that emit (give out) light are called luminous objects. Examples of luminous objects are:

• Sun
• Stars
• Electric bulbs
• Candles

Objects that do not emit light are termed non-luminous objects. The majority of objects fall into this category. We are able to see non-luminous objects when light reflects off them, into our eyes.

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