Physics Yr 9

  • The period of a wave is how long it takes to go through its full motion once.

  • The frequency is how many waves go by in a second.

A third feature is often used to describe waves.

  • The wavelength is the distance between one crest of a wave and the next crest.

Light is a type of energy that travels in waves. It is emitted by sources, such as the Sun or an electric light bulb. Any type of energy that is given off by objects as waves or particles is called radiation. Therefore, light is a type of radiation.

 

Scientific investigations have revealed that light waves are made up of both electric and magnetic fields. For this reason, scientists now define light as a type of electromagnetic radiation. As we'll see, this discovery has led to a wide range of invaluable technologies.

Light is just the range of electromagnetic waves that we humans can see. The full range of electromagnetic radiation is called the electromagnetic spectrum.

All electromagnetic waves travel at the same speed – the speed of light. This is incredibly fast. For example, light travels from the Moon to your eye in only 1.3 seconds!

The property that makes electromagnetic waves behave differently is their wavelength. This is related to how much energy they transfer from one place to another: the shorter the wavelength, the higher the energy. For example, X-rays have very short wavelengths and very high energies. In contrast, radio waves have long wavelengths and low energy.

Each type of electromagnetic wave is useful for different things. For example, doctors use X-rays to see inside our bodies while we often use microwaves to heat our food. These uses depend on the wavelength or the amount of energy transferred by each type of wave.

Depending on their wavelength, each type of electromagnetic radiation can be used for different purposes. For example, the short wavelengths of microwaves allow them to carry lots of information over several metres. This makes it perfect for your home Wi-Fi!

Some types of electromagnetic radiation can harm living things in particular ways. For example, some intense UV radiation can kill bacteria and viruses. This makes it useful for sterilizing drinking water, medical equipment or even your mobile phone!

So how do we see all of those objects that don't produce light?

To explain what happens when light interacts with objects, it helps to think of light rays that travel in straight lines.

Objects that don't produce their own light can only be seen because they reflect some of the light that hits them. Reflection is the bending of light as it bounces off a surface. We see objects when the reflected light enters our eyes, as shown in the diagram.

Instead of bouncing off an object's surface, a light ray might be absorbed. Absorption is the transfer of light energy into an object, where it is transformed into heat energy. In most cases, the amount of energy transformed is too small for us to feel as warmth.

Finally, a light ray might pass through an object without being absorbed. This is called transmission.

Light always travels through air at the same speed and in a straight line. When light hits an object, some of it bounces or reflects off the surface. The type of surface determines how the light reflects:

  • Uneven or rough surfaces reflect light rays in different directions. This scattering of light is called diffuse reflection.

  • In contrast, smooth, shiny surfaces reflect light rays in a regular pattern. This allows us to see a clear image reflected back at us and is called regular reflection. Mirrors are a great example of this.

The way a light ray reflects off a surface follows a simple pattern. The incoming ray is called the incident ray. To work out the direction of the reflected ray, we can think of a line at right angles to the surface. This line is called the normal. As shown in the diagram below:

  • The angle between the incident ray and the normal is called the angle of incidence.

  • The angle between the reflected ray and the normal is called the angle of reflection.

You may have noticed that there is a relationship between the angle of reflection and the angle of incidence. This relationship is called the law of reflection. It states that:

 

The angle of reflection is always equal to the angle of incidence. 

 

This law explains why the reflected rays have a regular pattern in the case of regular reflection.

Unless it encounters an object, light always travels through air at the same speed and in a straight line. However, when it travels from air into water or glass, it slows down slightly and changes direction. The bending of light as it passes from one material into another is called refraction.

Transparent objects with surfaces that refract light are called prisms. The amount of refraction depends on the material the prism is made out of and the wavelength of the light.

In a famous experiment, Sir Isaac Newton shone a beam of white light through a prism and discovered that it separated into a rainbow of colours. This observation made him realize that white light is made up of all the colours of the visible spectrum. The colours can be separated by a prism because they are refracted by different amounts.

The same effect produces rainbows when sunlight is refracted by water droplets in the atmosphere.

A lens is a curved piece of transparent glass or plastic that refracts light. Since a lens is curved on at least one side, light rays striking different parts of its curved surface change direction by different amounts.

Depending on the shape of the lens, the light rays either:

  • get further apart, or diverge

  • get closer together, or converge

A lens that is curved inwards and is thinner in the middle is called a concave lens.

 

Concave lenses cause parallel light rays to diverge, or spread out. Because the rays diverge in this way they appear to an observer to come from one point. This is called the focal point.

 

The distance from the focal point to the centre of the lens is called the focal length.

A lens that is curved outwards and is thicker in the middle is called a convex lens.

Convex lenses cause parallel light rays to converge, or get closer together. The point where parallel rays converge is called the focal point. Just as for concave lenses, the distance between the focal point and the centre of the lens is the focal length.

Telescopes and microscopes are excellent examples of how lenses are used by scientists to expand our knowledge.

  • Telescopes

are instruments that magnify images of very distant objects so we can see the details.

  • Microscopes

are instruments that magnify images of very small objects that are close to us.

Both of these devices have been used for centuries to explore the world around us.

 

Theoretical Concepts

1. Properties of a Wave

  • Wavelength (λ): The distance between two consecutive crests (or troughs) of a wave.

  • Amplitude: The maximum displacement from the rest position (basically how “tall” the wave is).

  • Frequency (f): How many complete waves pass a point in one second (measured in Hertz, Hz).

  • Crest: The highest point of a wave.

  • Trough: The lowest point of a wave.

2. Range of frequencies of electromagnetic radiation (EMR)

  • EMR spans an insanely wide range:

    • Radio waves (lowest frequency, longest wavelength)

    • Microwaves

    • Infrared

    • Visible light

    • Ultraviolet

    • X-rays

    • Gamma rays (highest frequency, shortest wavelength)

3. Link between frequency and wavelength

  • Frequency (f) and wavelength (λ) are inversely proportional.

  • Formula: c = f × λ (where c is the speed of light).

  • Higher frequency → shorter wavelength. Lower frequency → longer wavelength.

4. Visible light is a small section of the EM spectrum

  • Visible light is only 400–700 nm in wavelength (purple to red).

  • It’s the only part of the EM spectrum humans can see. Everything else is invisible without tech.

5. Behaviours of light at a surface

  • Absorption: Light energy gets soaked up (like black clothes in summer).

  • Transmission: Light passes through the material (like glass).

  • Reflection: Light bounces off (like a mirror).

6. Angle of incidence & reflection

  • Angle of incidence (i): Angle between the incoming ray and the normal (the perpendicular line to the surface).

  • Angle of reflection (r): Angle between the reflected ray and the normal.

7. Law of Reflection

  • Angle of incidence = Angle of reflection (i = r).

8. Density and refraction

  • Light bends when it changes medium (air → glass, water → air, etc).

  • If light goes into a denser medium (e.g., air → glass), it bends towards the normal.

  • If light goes into a less dense medium (e.g., glass → air), it bends away from the normal.

9. Refractive index calculation

  • Formula: n = sin(i) / sin(r)

    • i = angle of incidence, r = angle of refraction, n = refractive index.

10. Actual vs apparent depth

  • Refraction makes objects under water look shallower (closer) than they actually are.

  • The light bends away from the normal as it leaves the denser medium (water), so your eyes trace it back at the wrong angle.

11. Convex vs concave lenses

  • Convex lens (converging): Brings light rays together. Used in magnifying glasses, cameras, and to correct long-sightedness (hyperopia).

  • Concave lens (diverging): Spreads light rays apart. Used in peepholes, projectors, and to correct short-sightedness (myopia).

12. Basic eye anatomy (label + function)

  • Cornea: Transparent outer layer, starts bending light.

  • Pupil: Hole in the middle, lets light in.

  • Iris: Colored part, controls pupil size (like a camera shutter).

  • Lens: Focuses light onto the retina by changing shape.

  • Retina: Light-sensitive layer with photoreceptors (rods & cones).

  • Optic nerve: Sends electrical signals to the brain.

13. Physics of lenses in the eye

  • The eye’s lens is a convex lens.

  • It changes shape (accommodation) to focus light onto the retina.

14. Lenses correcting vision problems

  • Short-sighted (myopia): Can see near, but not far. Fixed with a concave lens (spreads rays so they focus further back).

  • Long-sighted (hyperopia): Can see far, but not near. Fixed with a convex lens (brings rays together sooner).

  • Astigmatism: Cornea/lens is irregularly shaped, needs specially curved lenses.