CIE IGCSE Physics (0625) Refraction of light Study Notes - New Syllabus
CIE IGCSE Physics (0625) Refraction of light Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Refraction of light
Key Concepts:
- Refraction
- Passage of Light Through a Transparent Material (At Boundaries Only)
- Critical Angle
- Internal Reflection and Total Internal Reflection (TIR)
- Optical Fibres in Telecommunications
Refraction
Refraction
Refraction occurs when a wave passes from one medium into another and changes speed. This causes the wave to change direction, unless it enters the new medium at 90° to the surface (along the normal).
- Normal: An imaginary line drawn perpendicular (90°) to the boundary between two media at the point where the wave strikes.
- Angle of Incidence (i): The angle between the incident ray and the normal when a wave enters a new medium.
- Angle of Refraction (r): The angle between the refracted ray and the normal in the second medium.
Experiment: Refraction of Light Through Transparent Blocks of Different Shapes
Aim: To observe and compare how light refracts through transparent blocks of different shapes (e.g., rectangular, semi-circular, triangular).
Apparatus:
- Ray box with a single slit
- Rectangular glass or plastic block
- Semi-circular block
- Triangular (prism-shaped) block
- Protractor
- Pencil and ruler
- Plain white paper
Procedure:
- Place the block on a sheet of paper and trace its outline.
- Use the ray box to shine a single light ray toward one face of the block at an angle (not 90°).
- Mark the points where the ray enters and exits the block.
- Remove the block and draw the incident ray, the refracted ray inside the block, and the emergent ray.
- Draw the normal at the point of incidence and measure the angles of incidence and refraction using a protractor.
- Repeat the procedure for the other shaped blocks (e.g. semi-circular and triangular).
Observations:
- In the rectangular block:
- The ray bends toward the normal when entering the block (from air to glass).
- It bends away from the normal when exiting (from glass to air).
- Emergent ray is parallel to the incident ray but displaced.
- In the semi-circular block:
- If the light enters at the curved surface along the radius, it enters at 90° and goes straight without bending.
- When it exits through the flat surface, it bends away from the normal.
- In the triangular (prism) block:
- The ray bends twice — once upon entering and once upon exiting.
- This causes the light to be deviated or dispersed if white light is used.
Conclusion:
- Refraction depends on the shape of the block and the angle at which light enters and exits.
- Different shapes result in different paths and deviations of the refracted ray.
Example :
A ray of light enters a rectangular glass block at an angle of 45° to the normal. It emerges on the other side. Explain how the direction of the ray changes as it enters and exits the block.
▶️ Answer/Explanation
When the ray enters the glass block from air, it slows down because glass is denser. This causes it to bend towards the normal.
Inside the glass, the ray travels in a straight line. Upon exiting back into air, it speeds up and bends away from the normal.
In a rectangular block, the emergent ray is parallel to the incident ray, but laterally displaced.
Conclusion: Refraction occurs at both surfaces, but the ray emerges in the same overall direction it entered, just shifted sideways.
Example :
A ray of light enters a semi-circular transparent plastic block through the curved surface, directed toward the flat face. Describe what happens to the ray inside the block and as it exits.
▶️ Answer/Explanation
The ray enters through the curved surface perpendicular to the tangent at the entry point. Because it enters along the radius, it meets the surface at 90°, so no refraction occurs at entry.
As the ray reaches the flat surface from inside the plastic (denser) to air (less dense), it bends away from the normal due to increased speed.
If the angle of incidence at the flat surface is large enough, total internal reflection may also occur.
Conclusion: Refraction only occurs at the flat surface, and the path depends on the angle at which the ray strikes the surface from inside.
Passage of Light Through a Transparent Material (At Boundaries Only)
Passage of Light Through a Transparent Material (At Boundaries Only)
When a light ray passes from one medium into a different transparent medium (e.g. from air to glass), its speed changes.This change in speed causes the light ray to refract (bend) at the boundary between the two media.
If light enters a denser medium (like air → glass):
- It slows down
- It bends toward the normal
If light exits into a less dense medium (like glass → air):
- It speeds up
- It bends away from the normal
Refractive Index (n):
The refractive index is a measure of how much a wave slows down when entering a new medium.
- It is defined as the ratio of the speed of the wave in the first medium to the speed in the second medium.
\( n = \dfrac{v_1}{v_2} \)
- \( v_1 \) = speed of wave in the first (less dense) medium
- \( v_2 \) = speed of wave in the second (denser) medium
Alternate Form (for light, using Snell’s Law):
\( n = \dfrac{\sin i}{\sin r} \)
- \( i \) = angle of incidence
- \( r \) = angle of refraction
Note: The higher the value of \( n \), the more the wave slows down and bends toward the normal.
At each boundary, the angle of incidence and angle of refraction can be measured and related using Snell’s Law:
\( n = \dfrac{\sin i}{\sin r} \)
No bending occurs if the ray hits the boundary at 90° to the surface (i.e., along the normal).
Note: The light continues in a straight line within each medium, only changing direction at the boundary.
Example:
When a ray of light passes from air into glass at an angle, it bends toward the normal. When it passes back into air, it bends away from the normal.
Why does this happen, and what does it tell us about the speed of light in different media?
▶️ Answer/Explanation
Light bends toward the normal in glass because it slows down (glass is denser than air).
When returning to air, the light speeds up and bends away from the normal.
This shows that the speed of light is different in different media, and refraction occurs due to this change in speed.
Conclusion: The more a ray bends, the greater the difference in speed between the two media.
Example:
A light ray enters a transparent plastic block from air. The angle of incidence is \( 45^\circ \), and the angle of refraction is \( 28^\circ \).
Calculate the refractive index of the plastic.
▶️ Answer/Explanation
We use Snell’s Law:
\( n = \dfrac{\sin i}{\sin r} \)
\( n = \dfrac{\sin 45^\circ}{\sin 28^\circ} \)
\( n = \dfrac{0.7071}{0.4695} \approx \boxed{1.51} \)
The refractive index of the plastic is approximately \( \boxed{1.51} \).
Critical Angle
Critical Angle
When light travels from a denser medium to a less dense medium (e.g., from glass to air), it bends away from the normal. As the angle of incidence increases, so does the angle of refraction until it reaches 90°.
- The critical angle is the angle of incidence in the denser medium at which the angle of refraction becomes exactly 90°.
- At this angle, the refracted ray travels along the boundary between the two media.
- Beyond this angle, total internal reflection occurs — all the light is reflected back into the denser medium.
Equation for the Critical Angle:
\( \sin c = \dfrac{1}{n} \)
- \( c \) = critical angle
- \( n \) = refractive index of the denser medium (assumes second medium is air or vacuum)
Rearranged to calculate the angle:
\( c = \sin^{-1} \left( \dfrac{1}{n} \right) \)
Example:
Light is travelling from glass (refractive index \( n = 1.52 \)) into air.
Calculate the critical angle for this glass-air boundary.
▶️ Answer/Explanation
Step 1: Use the critical angle formula
\( \sin c = \dfrac{1}{n} \)
\( \sin c = \dfrac{1}{1.52} \approx 0.6579 \)
Step 2: Take the inverse sine
\( c = \sin^{-1}(0.6579) \approx \boxed{41.1^\circ} \)
The critical angle is approximately \( \boxed{41.1^\circ} \).
Internal Reflection and Total Internal Reflection (TIR)
Internal Reflection and Total Internal Reflection (TIR)
Internal Reflection:
- Occurs when light travels from a denser medium to a less dense medium (e.g., glass to air).
- Some light is reflected inside the denser medium, and some is refracted out into the less dense medium.
Total Internal Reflection (TIR):
- If the angle of incidence is greater than the critical angle, the light ray is completely reflected inside the denser medium.
- No light is refracted; it’s all reflected back – this is called total internal reflection.
- Occurs only when:
- Light is travelling from a denser to a less dense medium
- The angle of incidence is greater than the critical angle
Experimental Example (Glass Block):
- Shine a light ray into a semicircular glass block using a ray box.
- Gradually increase the angle of incidence at the flat side.
You will observe:
- At small angles: partial reflection and refraction
- At the critical angle: refracted ray grazes the boundary (90°)
- Beyond that: total internal reflection , no refraction at all
Everyday Examples of Total Internal Reflection:
- Optical fibres: Light signals bounce inside the fibre by total internal reflection, allowing high-speed data transmission.
- Periscopes: Use angled prisms to reflect light using TIR instead of mirrors for a clearer image.
- Shimmering effect in water: Light inside the water reflects off the surface when viewed at shallow angles.
- Diamond sparkle: Due to a high refractive index, diamonds trap light and reflect it internally many times before it escapes.
Example :
A ray of light travels from water (refractive index \( n = 1.33 \)) into air. The angle of incidence is 30° at the water–air boundary. Explain what happens to the ray.
▶️ Answer/Explanation
Since the ray is travelling from a denser to a rarer medium (water to air), it bends away from the normal at the boundary.
At 30°, this angle is less than the critical angle for the water–air boundary (about 48.8°).
Hence, some light is refracted into the air, and some is reflected back into the water , this is called partial internal reflection.
Conclusion: Total internal reflection does not occur unless the angle of incidence exceeds the critical angle.
Optical Fibres in Telecommunications
Optical Fibres in Telecommunications
Optical fibres are thin, flexible strands of glass or plastic that transmit light over long distances.
- They work on the principle of total internal reflection (TIR) light reflects completely inside the core of the fibre with minimal energy loss.
How They Work:
- Light signals (usually lasers or LEDs) are sent into the fibre at an angle greater than the critical angle.
- The light reflects internally along the fibre core, bouncing from the inside walls with almost no loss of energy.
- The outer layer (cladding) ensures the light stays inside the core and reduces signal interference.
Uses in Telecommunications:
Optical fibres are used to carry digital data as pulses of light over long distances.
- They are commonly used in:
- Internet and broadband – high-speed data transmission
- Telephone networks – clearer and faster voice signals
- Cable TV – delivering high-definition digital signals
- Medical imaging – in endoscopes (non-telecom example)
Advantages:
- Very fast data transmission speeds
- Low signal loss over long distances
- Not affected by electromagnetic interference
- Secure (harder to tap than copper wires)
Example :
Light is travelling inside an optical fibre made of glass (\( n = 1.5 \)). It hits the internal wall at an angle of incidence of 65°. Explain whether total internal reflection occurs, and why optical fibres use this effect.
▶️ Answer/Explanation
For glass-air interface, the critical angle is about 42°.
Since the ray strikes the surface at 65°, which is greater than the critical angle, total internal reflection (TIR) occurs.
The light is completely reflected back inside the fibre and continues to bounce internally with no loss of energy through the surface.
Conclusion: Optical fibres use TIR to keep the light inside the fibre, even around bends, allowing efficient signal transmission over long distances.