When was the law of reflection discovered

For example, we want the light to bounce off a mirror or to pass through a piece of glass on its way from A to B.

Fermat's principle states that of all the possible paths the light might take, that satisfy those boundary conditions, light takes the path which requires the least amount of time. Discuss this with your fellow students in the discussion forum! Consider the speed of of person 1 on the sand and in the water. Which path takes the least amount time in situation A and in situation B. Light can travel faster in air than in water.

The qualitative arguments you make about the path that takes the least amount of time for the person near the sand-water interface in situations A and B also apply to the path of light light entering from air into water or reflecting from the air-water interface. That is Fermat's principle.

Fermat's principle leads automatically to the principle of ray reversibility in geometrical optics. It does not matter if you are going from A to B or B to A, the path that takes the least amount of time is the same. Reflection is the abrupt change in the direction of propagation of a wave that strikes the boundary between two different media. At least some part of the incoming wave remains in the same medium.

Note: In geometrical optics, angles are always measured with respect to the normal to the interface. Specular reflection occurs at smooth, plane boundaries. Then the plane tangent to the boundary is the boundary itself. Reflection at rough, irregular boundaries is diffuse reflection. The smooth surface of a mirror reflects light specularly, while the rough surface of a wall reflects light diffusely.

The reflectivity of a surface material is the fraction of energy of the incoming wave that is reflected by the surface. The reflectivity of a mirror is close to 1. How many times will the incident beam shown in the figure to the right be reflected by each of the parallel mirrors?

Refraction is the change in direction of propagation of a wave when the wave passes from one medium into another and changes its speed. Light waves are refracted when crossing the boundary from one transparent medium into another because the speed of light is different in different media. Assume that light waves encounter the plane surface of a piece of glass after traveling initially through air as shown in the figure to the right.

What happens to the waves as they pass into the glass and continue to travel through the glass? The speed of light in glass or water is less than the speed of light in a vacuum or air. Typical values for the index of refraction of glass are between 1. The distance between wave fronts will therefore be shorter in the glass than in air, since the waves travel a smaller distance per period T.

Now consider wave fronts and their corresponding light rays approaching the surface at an angle. We can see that the rays will bend as the wave passes from air to glass. The bending occurs because the wave fronts do not travel as far in one cycle in the glass as they do in air.

As the diagram shows, the wave front halfway into the glass travels a smaller distance in glass than it does in air, causing it to bend in the middle. Thus, the ray, which is perpendicular to the wave front, also bends. The situation is like a marching band marching onto a muddy field at an angle to the edge of the field.

The rows bend as the speed of the marchers is reduced by the mud. The amount of bending of the light depends on the angle of incidence and on the indices of refraction of glass and air, which determine the change in speed.

This is Snell's law , or the law of refraction. When light passes from one transparent medium to another, the rays bend toward the surface normal if the speed of light is smaller in the second medium than in the first. The rays bend away from this normal if the speed of light in the second medium is greater than in the first. The Mind and Quantum Mechanics. The Limitations of Science. List of symbols. Image Copyright. This states that light travels in straight lines and reflects from a surface at the same angle at which it hit it.

Light is reflected in the same way that a ball would bounce off of a frictionless surface, and so Euclid claimed that light travels in rays that are discrete, like atoms, not continuous, like waves. This may mean that some of the objects in our visual field will always remain unilluminated, and therefore undetected. Unlike Aristotle, Euclid thought that light is emitted in rays from the eye.

Figure 2. The Roman astronomer Ptolemy first tried to experimentally derive the law of refraction in the 2nd century CE. This is either because it slows down or because it speeds up. Ptolemy measured the angle that a beam of light hits a boundary, the angle of incidence, and the angle at which it leaves, the angle of refraction, through different mediums. He discovered that the angle of incidence is proportional to the angle of refraction, but could not derive the full equation.

Like Euclid, Ptolemy thought that light is emitted in rays from the eye. The Iraqi mathematician Ibn Sahl discovered the full law of refraction in Sahl showed that the angle of incidence is related to the angle of refraction using the law of sines.

Here v is velocity and n is the refractive index. The Dutch mathematician Willebrord Snellius rediscovered the sine law of refraction in The sine function shows how the angle inside a triangle changes as the lengths of its sides change.

The ancient Greek astronomer Hipparchus created the first documented table of sine functions before BCE. Sin, cos, and tan waves could not be represented graphically until after the invention of the Cartesian coordinate system by Descartes and the French mathematician Pierre de Fermat in The Iraqi mathematician Ibn al-Haytham also known as Alhazen was the first person to correctly describe how perception occurs in about when he proved that light enters, but is not emitted by, the eye.

Al-Haytham experimented with the laws of reflection and refraction using different shaped mirrors and lenses, and he accurately described how the eye functions as an optical instrument. He likened it to a camera obscura , a pinhole camera, and so suggested that images must also be inverted in the eye. This led him to suggest that vision occurs in the brain, rather than the eyes and that it is, therefore, subjective.

Science began to progress again in Europe after the Renaissance of the 12th century. This was mainly due to increased contact with the Islamic world. The German philosopher Theodoric of Freiberg also known as Dietrich of Freiberg explained how rainbows are formed in about Theodoric did this by experimenting with spherical flasks, which he filled with water to represent raindrops.

Theodoric showed that the light of the Sun is refracted, and then internally reflected, inside each drop. The set of all the raindrops that can be seen from this angle at once forms a cone pointing towards the Sun. Coloured light is not visible from any other angle, and so when you move, the rainbow moves with you.

This is why you will never be able to reach the end of a rainbow. In the case of a double rainbow, secondary bows are caused by double reflection inside the raindrop. Rainbows form when light is refracted and then internally reflected inside raindrops. Water and sound waves act the same way, and so this was an important finding for those that advocated for a wave theory of light. Before this, people had argued that the sharp boundaries created by shadows meant that light could not bend around corners in the same way that water or sound waves can.

Copyright Privacy Disclaimer Search Sitemap. I Pre 20th Century theories 1. Atoms and Waves 2. Reflection, Refraction, and Diffraction 3. Newton's theory of Light 4. Measuring the Speed of Light 5.

Origin of Quantum Mechanics 9. Development of Atomic theory