Grade 8 → Lighting and Optics ↓
Reflection - laws and applications in optical instruments
Introduction to reflection
Reflection happens when light hits a surface. It is similar to the way a ball bounces off a wall. When light hits a surface, some part of it goes back. This is why we are able to see ourselves in a mirror!
The study of light and its behaviour is called optics, which is an essential part of physics. Light travels in a straight line at a speed of about 300,000 kilometres per second (in a vacuum). When light hits an object, it behaves differently depending on the nature of the surface.
Understanding reflection
To understand reflection, consider a flat, smooth surface, such as a mirror. When light falls on this smooth surface, most of the light reflects back, and we can see images on the mirror's surface.
Here's a simple visual example that shows how light reflects off a surface:
Laws of reflection
There are two main laws of reflection in physics:
- The angle of incidence is equal to the angle of reflection.
- The incident ray, the reflected ray and the normal to the surface all lie in the same plane.
Let us explain what these mean:
Angle of incidence and reflection
The angle of incidence is the angle between the incoming light and a line perpendicular to the surface (this perpendicular line is called the 'normal'). The angle of reflection is the angle between the reflected light and the normal line. According to the first law, these angles are equal.
Angle of incidence = Angle of reflection
In the graphic you can see the angle of incidence (θi) and angle of reflection (θr) are equal.
Equal plane rule
The second law states that the light falling on a surface, the light reflecting off the surface, and the normal line all lie flat on the same imaginary plane. This means they are as flat as a piece of paper.
Imagine a table. If the incident ray, the reflected ray, and the normal all lie on this table, they will always be in the same imaginary plane.
Types of reflection
The reflection of light can vary, depending mainly on the type of surface:
Specular reflection
Reflection occurs on smooth surfaces like mirrors or still water, where the reflected rays remain parallel. This type of reflection allows us to see clear images in mirrors.
Diffuse reflection
Diffuse reflection occurs on rough surfaces like paper or stone, where the reflected light is scattered in many directions. We don't see a clear image, but the surface appears to be visible under the light.
Applications of reflection in optical instruments
Reflection plays an important role in various optical devices. Here are some common applications:
Mirror
Mirrors are one of the most straightforward applications of reflection. They are used in homes, cars, for decoration and in complex devices like telescopes. Mirrors give a clear image of everything in front of them because of reflection.
Periscope
A periscope is a device that allows one to look above or around something. It is commonly used on submarines. Light is reflected inside the periscope using mirrors at a 45-degree angle, allowing the submarine to see objects farther from the surface.
Telescope
Telescopes use mirrors to gather and focus light from distant objects. They allow astronomers to see distant galaxies and stars. Reflecting telescopes use a large curved primary mirror to gather light and reflect it to a focal point.
Camera
Cameras use lenses, prisms, and mirrors to direct light and focus it onto the film or sensor. Optical viewfinders in cameras use mirrors to reflect light from the lens to your eye, allowing you to see the scene as it will be captured.
Conclusion
The laws of reflection are simple but powerful principles that define how light interacts with surfaces. Understanding these laws helps us effectively design and use a variety of optical devices, ranging from everyday objects like mirrors to sophisticated instruments like telescopes and cameras.
Reflection on real life
During your day, try to observe how light reflects off of different surfaces. Notice how light behaves on smooth versus rough surfaces and how these reflections help you see your world more clearly. Understanding reflection not only empowers you in academics but also enhances your perception of the world!
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