Total Internal Reflection of Light
- Sometimes, when light is moving from a denser medium towards a less dense one, instead of being refracted, all of the light is reflected
- This phenomenon is called total internal reflection
- Total internal reflection (TIR) occurs when:
- The angle of incidence > the critical angle
- The incident material is denser than the second material
Refraction and Total Internal Reflection
Light is refracted when it passes from a more dense to a less dense medium when the critical angle is greater than the incident angle. Light is internally reflected when it passes from a more dense to a less dense material when the critical angle is less than the incident angle
Worked example
A glass cube is held in contact with a liquid and a light ray is directed at a vertical face of the cube. The angle of incidence at the vertical face is 39° and the angle of refraction is 25° as shown in the diagram. The light ray is totally internally reflected for the first time at X. 
Complete the diagram to show the path of the ray beyond X to the air and calculate the critical angle for the glass-liquid boundary.
Answer:
A correctly drawn diagram would look like this:
Step 1: Draw the reflected angle at the glass-liquid boundary
- When a light ray is reflected, the angle of incidence = angle of reflection
- Therefore, the angle of incidence (or reflection) is 90° – 25° = 65°
Step 2: Draw the refracted angle at the glass-air boundary
- At the glass-air boundary, the light ray refracts away from the normal
- Due to the reflection, the light rays are symmetrical on the other side
Step 3: Calculate the critical angle
- The question states the ray is “totally internally reflected for the first time” meaning that this is the lowest angle at which TIR occurs
- Therefore, 65° is the critical angle
Exam Tip
If asked to name the phenomena make sure you give the whole name – total internal reflection and not just TIR
Remember: total internal reflection occurs when light travels from a denser material to less dense material and ALL of the light is reflected.
If you are asked to explain what is meant by the critical angle in an exam, you can be sure to gain full marks by drawing and labelling the diagram shown in the notes above.
