Every student knows that light in a uniform transparent medium moves along a straight path. This fact allows us to consider many optical phenomena in the framework of the concept of a light beam. This article talks about the angle of incidence of the beam, and why it is important to know this angle.
Ray of light - micrometer electromagnetic wave
In physics, there are waves of various nature: sound, sea, electromagnetic, and some others. However, the concept of “beam” applies only to electromagnetic waves, of which the visible spectrum is a part. The word "ray" itself can be represented as a straight line connecting two points in space.
Light (as a wave) can be considered as a straight line, because each wave implies the presence of vibrations. The answer to this question is the meaning of the wavelength. So, for marine and sound length ranges from a few centimeters to tens of meters. Of course, such oscillations can hardly be called a ray. The wavelength of light is less than one micrometer. The human eye is not able to distinguish such vibrations, which is why it seems to us that we are seeing a direct ray.
For completeness, it is worth noting that a light beam is visible only when it begins to scatter on small particles, for example, in a dusty room or droplets of fog.
Where is it important to know the angle at which the beam hits an obstacle?
The phenomena of reflection and refraction are the most famous optical effects that a person encounters literally every day, when he looks at himself in the mirror or drinks a glass of tea, having first looked at the spoon in it.
The mathematical description of refraction and reflection requires knowledge of the angle of incidence of the beam. For example, the reflection phenomenon is characterized by the equality of the angle of reflection and incidence. If described from the side of the refraction process, the angle of incidence and the angle of refraction are related to each other through the functions of the sines and the refractive indices of the media (Snell's law).
The angle at which the light beam hits the interface between two transparent media plays an important role in considering the effect of internal total reflection in an optically denser material. This effect is observed only in the case of incidence angles that are greater than some critical value.
Geometric definition of the angle considered
It can be assumed that there is some surface that separates the two media. This surface can be a plane, as in the case of a mirror, or have a more complex shape, for example, a ribbed surface of the sea. Imagine a light beam incident on this surface. How to determine the angle of incidence of light? This is easy enough to do. The following is the sequence of actions that should be done to find the desired angle.
- First you need to determine the point of intersection of the beam with the surface.
- Perpendicular should be drawn through O to the surface under consideration. It is often called the normal.
- The angle of incidence of the beam is equal to the angle between it and the normal. It can be measured with a simple protractor.
Apparently, it is not difficult to find the considered angle. However, students often make a mistake by measuring it between the plane and the beam. It must be remembered that the angle of incidence is always counted from the normal, regardless of the shape of the surface and the medium in which it propagates.
Spherical mirrors, lenses and rays incident on them
Knowledge of the properties of the angles of incidence of certain rays is used in the construction of images in spherical mirrors and thin lenses. To construct such images, it is enough to know how two different beams behave when interacting with these optical devices. The intersection of these rays determines the position of the image point. In the general case, you can always find three different beams whose course is exactly known (the third beam can be used to verify the correctness of the constructed image). These rays are named below.
- Running parallel to the main optical axis of the device. After reflection or refraction, it passes through the focus.
- A beam going through the focus of the device. It is always reflected or refracted parallel to the main axis.
- Going through the optical center (for a spherical mirror it coincides with the center of the sphere, for a lens it is inside it). Such a beam does not change its trajectory.
Above the figure shows the scheme for constructing images for different options for the location of the object relative to thin lenses.