What is the difference between reflection and refraction?

Reflection is when light bounces off a surface, obeying the law of reflection (angle of incidence = angle of reflection). Refraction is when light changes direction as it passes from one medium to another due to a change in speed. For example, a mirror causes reflection, while a glass prism causes refraction.

How do I remember the order of the electromagnetic spectrum?

A common mnemonic is 'Rabbits Mate In Very Unusual eXpensive Gardens' for Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma. Remember that frequency increases from left to right, and wavelength decreases.

What is total internal reflection and how is it used?

Total internal reflection occurs when light travels from a denser to a less dense medium at an angle greater than the critical angle, causing all light to reflect back inside the denser medium. It is used in optical fibres to transmit data over long distances with minimal loss, and in endoscopes to see inside the body.

Why does a straw look bent in a glass of water?

This is due to refraction. Light from the straw travels from water (denser) to air (less dense), causing it to bend away from the normal. Our brain interprets the light as coming from a straight line, making the straw appear bent at the water's surface.

What are the dangers of ultraviolet radiation?

Ultraviolet (UV) radiation has higher energy than visible light and can damage skin cells, leading to sunburn and increasing the risk of skin cancer. It can also cause eye damage like cataracts. However, it is also used in sterilisation and fluorescent lamps.

How do I calculate the refractive index?

The refractive index (n) is calculated using Snell's law: n = sin i / sin r, where i is the angle of incidence in air (or vacuum) and r is the angle of refraction in the material. For example, if light enters glass at 30° and refracts at 19°, n = sin(30°)/sin(19°) ≈ 0.5/0.3256 ≈ 1.54.

Topic 5 – Light and the electromagnetic spectrum

EDEXCEL

GCSE

This topic covers the fundamental properties of waves, including the distinction between transverse and longitudinal waves and the transfer of energy without matter. It also explores wave characteristics such as frequency, wavelength, amplitude, and velocity, alongside the effects of reflection, refraction, transmission, and absorption at material interfaces.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

Topic 5 – Light and the electromagnetic spectrum explores the nature of light as both a wave and a particle, and its interactions with matter. You'll learn about reflection, refraction, and the dispersion of white light, as well as the full electromagnetic spectrum from radio waves to gamma rays. This topic is fundamental to understanding how we see the world, how technologies like Wi-Fi and X-rays work, and how energy is transferred across space.

In the Edexcel GCSE Physics course, this topic builds on your knowledge of waves from Topic 4 (Waves). It connects to real-world applications such as fibre optics, medical imaging, and communication systems. Mastering this topic will help you explain phenomena like why a straw looks bent in water, how rainbows form, and why some telescopes are placed in space.

By the end of this topic, you should be able to draw ray diagrams for reflection and refraction, calculate refractive index using Snell's law, recall the order of the electromagnetic spectrum, and describe the uses and dangers of each type of radiation. This knowledge is essential for understanding more advanced concepts in physics and for answering exam questions that require both recall and application.

Key Concepts

Core ideas you must understand for this topic

→Reflection: The law of reflection states that the angle of incidence equals the angle of reflection, measured from the normal. This applies to plane mirrors and is used in periscopes and safety mirrors.
→Refraction: When light passes from one medium to another, it changes speed and direction. The refractive index (n) is defined as n = sin i / sin r, where i is the angle of incidence and r is the angle of refraction. A higher refractive index means light slows down more.
→Total internal reflection: Occurs when light travels from a denser to a less dense medium at an angle greater than the critical angle. This is used in optical fibres for communication and endoscopes.
→The electromagnetic spectrum: A continuous range of waves from radio waves (longest wavelength, lowest frequency) to gamma rays (shortest wavelength, highest frequency). All travel at the speed of light in a vacuum (3 × 10⁸ m/s).
→Dispersion: White light can be split into its component colours (red, orange, yellow, green, blue, indigo, violet) using a prism. This happens because different colours refract by different amounts.

What You Need to Demonstrate

Key skills and knowledge for this topic

Waves transfer energy and information without transferring matter
Distinction between longitudinal and transverse waves
Use of wave speed equation v = f × λ
Use of wave speed equation v = x / t
Refraction at a boundary involves a change in speed and direction
Ultrasound and infrasound definitions and applications
Relationship between frequency, wavelength, and velocity when changing media

Marking Points

Key points examiners look for in your answers

Waves transfer energy and information without transferring matter
Distinction between longitudinal and transverse waves
Use of wave speed equation v = f × λ
Use of wave speed equation v = x / t
Refraction at a boundary involves a change in speed and direction
Ultrasound and infrasound definitions and applications
Relationship between frequency, wavelength, and velocity when changing media

Examiner Tips

Expert advice for maximising your marks

💡Always show working for calculations, especially when rearranging the wave speed equation
💡Use a ruler for drawing ray diagrams to ensure accuracy
💡Be precise with definitions of frequency and wavelength
💡Remember that the frequency of a wave remains constant when it changes medium
💡Always draw ray diagrams with a ruler and label the normal, incident ray, reflected/refracted ray, and angles. Use a protractor to measure angles accurately. Examiners look for neat, labelled diagrams.
💡When answering questions about the electromagnetic spectrum, be specific about the properties (wavelength, frequency, speed) and give an example of a use and a danger for each type. For example, 'X-rays have high frequency and are used for medical imaging, but they can cause cancer if overexposed.'
💡For refraction calculations, remember to use the formula n = sin i / sin r. If the light is going from a less dense to a denser medium, the angle of refraction is smaller than the angle of incidence. Always check your calculator is in degree mode.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing the direction of particle oscillation with the direction of energy transfer
Incorrectly stating that waves transfer matter
Failing to convert units (e.g., kHz to Hz) before using the wave speed equation
Misinterpreting the relationship between frequency and wavelength in different media
Misconception: Light always travels in straight lines. Correction: Light travels in straight lines in a uniform medium, but it can bend (refract) when entering a different medium or be diffracted around obstacles.
Misconception: The electromagnetic spectrum is ordered by frequency from low to high: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Correction: The correct order is radio, microwave, infrared, visible, ultraviolet, X-ray, gamma (increasing frequency). A common mnemonic is 'Rabbits Mate In Very Unusual eXpensive Gardens'.
Misconception: Total internal reflection only happens in glass. Correction: It can occur in any transparent material where light travels from a higher refractive index to a lower one, such as from water to air.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Topic 4 – Waves: Understanding wave properties (frequency, wavelength, amplitude, speed) and the wave equation v = fλ is essential for grasping the electromagnetic spectrum.
•Basic trigonometry: You need to know how to use sine, cosine, and tangent for angles, especially for Snell's law calculations.
•Energy transfer: A basic understanding of how energy is transferred by waves (e.g., infrared radiation transferring thermal energy) helps in understanding applications.

Likely Command Words

How questions on this topic are typically asked

Calculate

Describe

Explain

State

Compare

Ready to test yourself?

Practice questions tailored to this topic

Topic 5 – Light and the electromagnetic spectrum

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in EDEXCEL GCSE Physics

Topic 1 – Key concepts of physics

Topic 10 – Electricity and circuits

Topic 11 – Static electricity

Topic 12 – Magnetism and the motor effect

How to Revise Topic 5 – Light and the electromagnetic spectrum — Edexcel GCSE Physics

Examiner Tips for Topic 5 – Light and the electromagnetic spectrum

Common Mistakes in Topic 5 – Light and the electromagnetic spectrum

Key Marking Points

Topic 5 – Light and the electromagnetic spectrum

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between reflection and refraction?

How do I remember the order of the electromagnetic spectrum?

What is total internal reflection and how is it used?

Why does a straw look bent in a glass of water?

What are the dangers of ultraviolet radiation?

How do I calculate the refractive index?

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in EDEXCEL GCSE Physics

Topic 1 – Key concepts of physics

Topic 10 – Electricity and circuits

Topic 11 – Static electricity

Topic 12 – Magnetism and the motor effect