Understanding Light Travel: A Worksheet Guide

Light is a form of energy that travels in straight lines at a speed of 300,000,000 meters per second. We see objects when light bounces off them and enters our eyes. Some objects, like the sun, create light and are called luminous objects. Non-luminous objects, like the moon, reflect light from luminous objects, making them visible. Understanding the properties of light, such as speed, direction, and how shadows are formed, is crucial for various scientific and practical applications.

What is light?

Light is electromagnetic radiation that can be detected by the human eye. It is a form of energy that travels in waves in straight lines. Light travels at an incredibly fast speed of 299,792,458 metres per second, or about 186,282 miles per second. This speed is so fast that a beam of light could travel around the equator of the world over seven times in one second.

Light is essential for life on Earth. It is our primary means of perceiving the world around us. Light from the Sun warms the Earth, drives global weather patterns, and makes photosynthesis possible. Light is also important for various scientific applications, such as studying the composition of distant stars and galaxies and observing microscopic physiological processes within living cells.

Visible light, or the light that humans can see, is only a small part of the electromagnetic spectrum. It ranges from about 400 nanometres for violet light to 700 nanometres for red light. Radiation shorter than 400 nanometres is called ultraviolet light, while radiation longer than 700 nanometres is called infrared light.

Light can be described as a stream of photons, which are massless packets of energy that travel with wave-like properties at the speed of light. Photons are the smallest quantity of energy that can be transported, and the realisation that light travelled in these discrete units was the origin of Quantum Theory.

How does light travel?

Light is a form of energy that travels in a wave. It moves in straight lines at an incredibly fast speed of 300,000,000 meters per second. To put that into context, a beam of light could circle the Earth's equator over seven times in just one second.

Light is essential for vision; we see objects when light bounces off them and enters our eyes. Objects that emit light are called luminous objects, like the Sun. We see non-luminous objects, like the Moon, when they reflect light from luminous objects, and that reflected light reaches our eyes.

Light travels faster than sound, as we can observe in the gap between seeing lightning and hearing thunder, or between dropping an object and hearing it hit the ground. It takes light 8.5 minutes to travel from the Sun to Earth, so when we look at the Sun, we see it as it was 8.5 minutes ago.

When we shine a laser, we can observe that light travels in a straight line. This straight-line travel also creates shadows, as light cannot pass through certain objects, so a shadow appears on the opposite side of the object to the light source.

How is light different from sound?

Light and sound are both forms of wave motion, but they have distinct characteristics and behaviours.

Firstly, they differ in velocity. Sound waves move through air at roughly 1,100 feet per second, while light waves travel through air and empty space at approximately 186,000 miles per second, or 300,000,000 metres per second. This difference in speed is evident in everyday experiences: we see lightning before hearing thunder, and we see a book hit the floor before we hear the resulting thud.

Secondly, sound waves are longitudinal, meaning they require a medium, such as air, for propagation. This is why sound cannot travel in space: there are no air molecules for the sound waves to move through. Light waves, on the other hand, are transverse and do not need a medium to travel. Light waves can move through empty space, and they do so by travelling in straight lines.

Additionally, sound waves are composed of alternate compressions and expansions of matter, whereas light waves are composed of electric and magnetic fields which propagate perpendicular to each other. Sound waves are also dependent on the density of the medium through which they travel: they move faster in denser substances. Light waves behave in the opposite manner, travelling slower through denser media. For example, light moves about one-third slower in water than in air.

Lastly, frequency affects both sound and light, but in different ways. In sound waves, frequency determines pitch: a slow vibration creates a low note, while a rapid vibration produces a higher note. In light waves, frequency determines colour: violet light is at the high-frequency end of the light spectrum, while red light is at the low-frequency end.

What are luminous objects?

Luminous objects are those that emit light by themselves. They are sources of light and are visible because the light they emit enters our eyes. The Sun, for instance, is a luminous object, as are the stars. We see the Moon, but it is not a luminous object because it is made of rock and only reflects the light from the Sun. The Moon is, therefore, a non-luminous object.

Luminous intensity is the measure of a light source's ability to emit light energy, either in general or in a particular direction. It is measured in candelas. Luminous objects are the cause of the sensation of light. They are visible because they emit light themselves.

Examples of luminous objects include the Sun, the stars, flames from candles, electric bulbs, torches, tube lights, and so on. The Sun is a natural luminous object, a near-perfect ball of hot plasma radiating energy as visible light. Stars are also natural luminous objects, consisting of a luminous spheroid of plasma that is visible to the naked eye at night due to the light they emit.

Man-made luminous objects include electric bulbs, torches, and tube lights. These objects are visible because they emit light, which then enters our eyes.

How are shadows formed?

Light is a form of energy that travels in waves in straight lines. We need light to see; when light bounces off an object and enters our eyes, we are able to see that object. Light can be produced by sources of light, which we call luminous objects, such as the Sun. Non-luminous objects, like the Moon, are visible to us because they reflect light from luminous objects.

Shadows are formed when light is blocked by an opaque object. In the absence of light, darkness is formed, which we call a shadow. This can be observed when we shine a laser ray (a very narrow beam of light) and notice that light travels in a straight line. When an opaque object is placed in the path of the light, the light travels sideways of the object, and a shadow is formed on the opposite side of the light source.

The formation of shadows requires two factors: light and an opaque object. Light travels in a straight line, and when it reaches an opaque object, it cannot pass through, so it travels around the object, creating a shadow. Transparent objects, on the other hand, allow light to pass through them unhindered, while translucent objects permit partial passage of light.

The characteristics of a shadow, such as its size, shape, and depth, depend on several factors. Firstly, the source of light influences the sharpness of the shadow. Bright light sources, like direct sunlight, produce sharp shadows, while dimmer light sources result in blurred shadows. Secondly, the position of light affects the length of the shadow. When light falls at a 90-degree angle perpendicular to the object, the shadow is shorter, and as the angle between the light beam and the object increases, the shadow lengthens. Lastly, the size and shape of the object determine the boundaries of the shadow. For instance, a ball will cast a circular shadow, while a rectangular box will create a shadow with a rectangular shape.

Frequently asked questions