Light – Reflection and Refraction · Lesson 8 of 15
Refraction of Light
“The pencil is perfectly straight; light is the one causing the drama.”
• Explain refraction as a change in direction caused by changing speed. • Interpret familiar apparent-displacement effects. • Distinguish normal incidence from oblique incidence. • Relate apparent depth to refracted rays reaching the eye. • Use simple activities to gather evidence for refraction.
A pencil partly immersed in water appears broken at the surface, and a coin under water seems closer than it really is. These are not changes in the objects. They arise because light changes direction while crossing between transparent media.
The change in direction of light when it passes obliquely from one transparent medium to another because its speed changes.
Within each uniform medium, the ray continues along a straight line. The change occurs at the boundary. When the ray is incident along the normal, its speed changes but its direction does not, because there is no sideways asymmetry. Oblique incidence produces visible bending.
Apparent Position
Light from the submerged part of a pencil bends as it leaves water and enters air. The eye assumes that the arriving rays travelled straight throughout and traces them backward. Their apparent origin lies above the actual submerged point, so the pencil appears displaced. The bottom of a pond, printed letters under glass and a coin in water appear raised for the same reason.
Problem
A coin hidden by the rim of an empty bowl becomes visible when water is added without moving the coin or observer. Explain.
- 1.Before water is added, straight rays from the coin are blocked by the rim.
- 2.After water is added, rays from the coin refract away from the normal as they pass from water to air.
- 3.Some refracted rays now travel over the rim and reach the observer.
- 4.The eye traces them backward, so the coin appears raised and becomes visible.
Dependence on the Media
The amount of bending is not the same for every pair of media. Replacing water with kerosene, or glass with plastic, changes the speed ratio and therefore the direction of the refracted ray. This is why apparent displacement differs with material.
Activity
Place a coin at the bottom of a bucket of water and try to pick it up while viewing from an angle. The first attempt often misses because the hand is guided toward the apparent raised position rather than the actual coin. Repeat and compare the apparent and actual locations.
Making a Hidden Coin Visible
Place a coin in a shallow bowl and move back until it is just hidden by the rim. Ask someone to pour water gently without disturbing the coin. The coin reappears because refracted rays can now reach the eye. Keep the eye fixed so the change cannot be attributed to movement.
Observing a Line Through Glass
Draw a thick straight line and place a glass slab obliquely over it. Viewed from the side, the segment under the slab appears shifted at the boundaries. If the edge is normal to the line, the sideways shift is less evident. Viewed from above, the line appears raised. These observations connect apparent position with the direction from which rays emerge.
Refraction does not mean that light follows a curved path throughout a uniform material. In this treatment, each segment is straight; direction changes at the interface.
Quiz
What is the immediate cause of refraction at a boundary?
Why does a submerged coin appear raised?
What happens to direction at normal incidence?
Why can different liquids produce different apparent shifts?
Where does bending occur in the ray model?
Practice Problems
- Explain the apparently bent pencil. Solution: Rays from the submerged part refract at the water-air boundary. The eye traces them backward to a raised apparent position, misaligning that part with the portion in air.
- Why is no directional bending seen for normal incidence? Solution: The ray meets the boundary symmetrically along the normal. Its speed changes, but its path continues along the same line.
- Predict whether the apparent position changes if water is replaced by another liquid. Solution: Yes. Different speed ratios produce different refraction and therefore different apparent displacement.
- Design a controlled hidden-coin observation. Solution: Fix the coin, bowl and eye position; record visibility when empty; add water without disturbing anything; compare. Only the medium changes.
- A student says the underwater part of a pencil physically bends. Give evidence against this. Solution: Remove the pencil or view it without crossing the interface; it is straight. The displacement changes with viewing medium and angle, showing an optical effect.
Key Takeaways
• Refraction occurs when light crosses media and its speed changes. • Visible bending requires oblique incidence. • At normal incidence the direction remains unchanged. • The eye locates an object by tracing arriving rays backward. • A submerged object commonly appears raised when viewed from air. • The amount of apparent displacement depends on the media involved.