Everything you see must reflect light in order to be visible. The bottom line? Without light, there won’t be sight. How can our eyes see things, then? Well, it’s mostly because light from an object travels through space, and into our eyes. Once the light reaches our eyes, it signals our brain to detect its appearance.
Yet, while we mostly see things for what they are, why do objects look odd when we view it from above the water? Well, the answer lies in the way light moves! You can understand more about this and other natural phenomena by learning about the reflection and refraction of light.
To describe the various movements of light, we use the following terminologies:
- Incident ray: An incoming light ray
- Reflected ray: A light ray moving away, or bouncing away from a surface
- Normal: An imaginary line that is perpendicular to the surface.
- Angle of incidence: The angle is incidence is measured between the incident ray and the surface normal.
- Angle of reflection: The angle of reflection is the angle between the reflected ray and the surface normal.
- Angle of refraction: The angle of refraction is the angle between the refracted ray and the surface normal.
Reflection is the change in direction of light when it hits an object and bounces back. Based on the law for the reflection of light, the angle of reflection is always equal to the angle of incidence for light reflection on smooth surfaces.
When light is reflected off a smooth surface at a definite angle, it is called specular reflection – you can visualise this with a mirror, or a still pond that reflects its surroundings almost perfectly. For rough surfaces, light reflects off in a myriad of angles, causing what is known as diffuse reflection – think of it as when the still pond is disturbed by a ripple, and the reflection becomes marred.
Refraction is the change of direction of light when it passes through one medium into another, caused by a change in density between the substances. Refraction of light is what happens when you shine a light onto the surface of water – the light seems to bend once it enters the water!
For physics calculations, it is important to remember that at the point of incidence, the incident ray, refracted ray and normal lie in the same plane.
Refractive index is a number that indicates how fast light travels through a medium. A medium in which the light bends more towards the normal, is described as more optically dense, with a high refractive index.
Refractive index is usually described as the ratio of the speed of light in a vacuum, c, to its speed in a specific medium, v. As such, you can relate the terms with the equation: c/v = n.
You can also use the angle of incidence and angle of refraction to find out the refractive index of a medium. The angle of incidence and angle of refraction of a medium is related by the equation sin i/sin r = n, in the case where light is travelling from air to a denser medium.
Total internal reflection
Total internal reflection refers to a complete reflection of light at the interface between an optically denser to less dense medium.
Think of it this way: Imagine a fish tank filled with water. If you shine a laser from within the fish tank, up towards the surface (interface between the mediums), some light will refract, and some light will reflect back into the water. However, at various angles of incidence, the amount of light refracted and reflected will differ. When all the light is fully reflected back into the water, that is termed as total internal reflection.
Thus, for total internal reflection to occur, these conditions must be fulfilled:
- The light travels from a more optically dense medium to an optically less dense medium, such as light travelling from water to air.
- The angle of incidence must be greater than the critical angle. The critical angle can be defined as the angle of incidence where the angle of refraction is 90°. It differs across various mediums.
Applications of total internal reflection
Total internal reflection is not just a cool phenomenon to know. It has numerous practical applications in technology as well! In fact, optical fibres are one area in which total internal reflection becomes very important. The principle of total internal reflection helps optical fibres transmit light (and information) across long distances without significant losses.
Adding to that, the glass threads used for optic fibres are very compact and affordable, making it an excellent tool for channelling telephone and internet signals. So, even as you are reading this article online now, you have total internal reflection to thank!
Learning about the reflection and refraction of light can be very exciting, as you can find plenty of real-life examples and applications! However, it can surely get confusing when you need to carry out calculations and solve problems relating to these concepts.
Not to worry, as you can always look to your physics tutor for help! If you are enrolled in a physics tuition class, you can be assured you will get plenty of practice and guidance so that you will be a master at the topic in no time!