Investigating Light

This lesson covers the core practicals and investigative skills related to light, as required by the Edexcel GCSE Physics specification (1PH0), Topic 5: Light and the Electromagnetic Spectrum. You need to know how to investigate refraction through a glass block, how to determine the focal length of a converging lens, and how to draw accurate ray diagrams for the exam.

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Core Practical: Investigating Refraction Through a Glass Block

This experiment investigates how light refracts when it passes through a rectangular glass block.

Apparatus

• Ray box (single slit) and power supply
• Rectangular glass block
• Sheet of white paper
• Protractor
• Ruler
• Pencil
• Pins (optional — for the pin method)

Method

1. Place the glass block on a sheet of white paper and draw around its outline with a pencil.
2. Remove the glass block and draw a normal (at 90° to one of the longer sides) at the point where you will shine the light.
3. Replace the glass block on the outline.
4. Shine the ray box so that a single ray of light hits the glass block at the normal. Mark the angle of incidence as 0° initially.
5. Set the angle of incidence to a chosen value (e.g. 10°, 20°, 30°, 40°, 50°, 60°).
6. Mark the position of the incident ray with two pencil dots and the position of the emergent ray (the ray leaving the other side of the block) with two pencil dots.
7. Remove the glass block and use a ruler to draw the incident ray, the refracted ray inside the block (connecting the entry and exit points), and the emergent ray.
8. Draw the normal at the point where the ray enters the block.
9. Use a protractor to measure the angle of incidence (i) and the angle of refraction (r) — both measured from the normal.
10. Repeat for several different angles of incidence (at least 5 different angles).

Recording Results

Angle of Incidence (i) / °	Angle of Refraction (r) / °	sin i	sin r
10
20
30
40
50
60

Calculating Refractive Index

For Higher tier, calculate the refractive index using:

$n = \frac{\sin i}{\sin r}$n=sinrsini​

You can also plot a graph of sin i (y-axis) against sin r (x-axis). The gradient of the straight line of best fit gives the refractive index of the glass block.

flowchart TD
    A["Set up ray box and glass block<br/>on white paper"] --> B["Draw around glass block"]
    B --> C["Draw the normal at the<br/>point of incidence"]
    C --> D["Shine ray at chosen<br/>angle of incidence"]
    D --> E["Mark incident ray and<br/>emergent ray with dots"]
    E --> F["Remove block, draw rays,<br/>measure angles i and r"]
    F --> G["Repeat for 5+ different<br/>angles of incidence"]
    G --> H["Calculate sin i / sin r<br/>for each pair of angles"]
    H --> I["Plot sin i vs sin r<br/>Gradient = refractive index"]

    style A fill:#2c3e50,color:#fff
    style I fill:#27ae60,color:#fff

Key Observations

• The angle of refraction is always less than the angle of incidence (when light enters glass from air) — the light bends towards the normal.
• At 0° (along the normal), there is no change in direction — the light passes straight through.
• The emergent ray is parallel to the incident ray but laterally displaced (shifted sideways).
• The refractive index should be approximately 1.50 for standard glass.

Sources of Error and Improvements

Source of Error	How to Improve
Difficulty seeing the exact position of rays	Use a darkened room to make the light rays more visible
Measuring angles inaccurately	Use a large protractor and measure from the centre of the normal carefully
Parallax error when marking ray positions	Use pins and view them from directly above; ensure the ray box is positioned carefully
Glass block not placed exactly on the outline	Draw around the block carefully and replace it precisely
Ray box produces a wide beam instead of a thin ray	Adjust the slit or use a narrower slit to produce a sharper ray
Only a few data points collected	Use at least 5–6 different angles and repeat each measurement to calculate a mean

Exam Tip: In 6-mark practical questions, always describe the method step by step, mention what you measure, state the equation you would use, and explain how to reduce errors. Include repeats and state you would calculate a mean value for the refractive index.

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Core Practical: Investigating the Focal Length of a Converging Lens

This experiment determines the focal length of a converging (convex) lens.

Method 1: Using a Distant Object

The simplest method:

1. Hold the converging lens and point it towards a distant object (e.g. a window, tree, or building far away).
2. Hold a white screen (piece of card) behind the lens.
3. Move the screen backwards and forwards until a sharp, clear image of the distant object is formed on the screen.
4. Measure the distance from the centre of the lens to the screen using a ruler or metre rule.
5. This distance is the focal length (f) of the lens, because light from a very distant object arrives as essentially parallel rays, and these converge at the focal point.
6. Repeat the measurement three times and calculate the mean focal length.

Exam Tip: For the distant object method, the object must be genuinely far away (at least across a large room or outside a window). If the object is too close, the image distance will not equal the focal length.

Method 2: Using a Lamp at Varying Distances (Lens Equation Method — Higher)

A more detailed method using the thin lens equation:

$\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$f1​=u1​+v1​

Where:

• f = focal length
• u = object distance (distance from object to lens)
• v = image distance (distance from lens to screen)

1. Set up a lamp (illuminated object), the converging lens, and a white screen on an optical bench or ruler.
2. Place the object at a known distance (u) from the lens.
3. Move the screen until a sharp image is formed.
4. Measure the image distance (v).
5. Repeat for several different object distances.
6. For each pair of u and v, calculate 1/u and 1/v, then calculate f using the equation above.
7. Calculate the mean focal length from all your results.

Recording Results (Method 2)

Object Distance (u) / cm	Image Distance (v) / cm	1/u / cm⁻¹	1/v / cm⁻¹	Focal Length (f) / cm
40.0
35.0
30.0
25.0
20.0

Sources of Error and Improvements

Source of Error	How to Improve
Difficult to judge when the image is sharpest	Move the screen slowly and mark the position of the sharpest image; have another person check
Measuring distances inaccurately	Use a metre rule clamped to the bench; measure from the centre of the lens
Parallax error in reading the ruler	Read the ruler at eye level, directly perpendicular to the scale
Object is not far enough away (Method 1)	Ensure the object is at least several metres away (ideally outside)
Lens is not perpendicular to the principal axis	Ensure the lens is mounted vertically and aligned with the light source

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Drawing Accurate Ray Diagrams in the Exam

Ray diagrams are a common source of marks in the exam. Follow these rules to maximise your marks:

General Rules

1. Always use a ruler — freehand lines will lose marks.
2. Always use a protractor when measuring angles (for reflection and refraction diagrams).
3. Draw the normal as a dashed line at 90° to the surface or boundary.
4. Label all rays: incident ray, reflected ray, refracted ray, normal.
5. Add arrows on rays to show the direction of light travel.
6. Measure angles from the normal, not from the surface.
7. For lens diagrams, mark F and 2F on both sides of the lens.
8. Use at least two of the three standard rays to locate the image.
9. Mark the image position clearly and state whether it is real or virtual.
10. If the image is virtual, show the diverging rays extended backwards as dashed lines.

Common Ray Diagram Mistakes

Mistake	Correction
Drawing freehand lines	Always use a ruler
Measuring angles from the surface	Measure from the normal
Forgetting to draw the normal	Always draw a dashed normal line at 90°
Not adding arrows on rays	Arrows must show the direction of light
Incorrect labelling	Label all angles, rays, and image position
Not marking F and 2F on lens diagrams	Mark these on both sides of the lens
Drawing thick/blurry lines	Use a sharp pencil and draw thin, clear lines

flowchart TD
    A["Start your ray diagram"] --> B["Draw the surface or lens"]
    B --> C["Mark F and 2F<br/>(for lens diagrams)"]
    C --> D["Draw the normal<br/>(dashed line, 90° to surface)"]
    D --> E["Draw incident ray<br/>with arrow, using ruler"]
    E --> F["Apply the rule<br/>(reflection: i = r<br/>refraction: towards/away from normal<br/>lens: use standard rays)"]
    F --> G["Draw the refracted/<br/>reflected ray with arrow"]
    G --> H["Label angles, rays,<br/>image position (real/virtual)"]

    style A fill:#2c3e50,color:#fff
    style H fill:#27ae60,color:#fff

Exam Tip: In the exam, if you are asked to "complete the ray diagram," you typically need to draw 2 rays, find where they cross (the image), and describe the image. Even if the diagram looks roughly right, you will lose marks if you do not use a ruler, if angles are not accurate, or if labels are missing. Take your time and be precise.

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Worked Example — Calculating Refractive Index from Experimental Data

In a glass block experiment, a student measures the following angles:

Angle of Incidence (i) / °	Angle of Refraction (r) / °
20	13
30	19
40	25
50	31
60	35

Calculate the refractive index using the results for i = 40° and r = 25°.