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  • Writer's pictureKristjan Salev


Updated: Jul 15, 2022

How do smartphone screen magnifiers work?

A smartphone screen magnifier is a device that magnifies the image or text on the screen and makes it more visible to us. It's good to use, for example, when watching movies or reading small print without having to zoom in all the time. Likewise, it is a great tool for people with poor eyesight. This device is a frame or box in which the smartphone is placed at a certain angle and at a certain distance from the magnifying screen. It's one big magnifying glass, and we should investigate how one magnifying glass magnifies the image, using geometrical optics.

What is geometrical optics?

One model used in optics is called ray optics or geometrical optics, which describes light propagation in terms of rays. As a ray of light, we can imagine a line in space that shows the direction in which light energy is propagating. In this model, we also do not take into account the wave properties of light, which makes our task much easier.

Smartphone screen magnifier as a convex lens

Lenses can generally be divided into two major groups: convex lenses and concave lenses. Depending on the type of lens, the light passing through it can travel along very different trajectories and produce images of different sizes and orientations. The magnifying screen of the smartphone is one big convex lens, which means that the surfaces of this lens are slightly outwardly curved and it works like an ordinary magnifying glass.

smartphone screen magnifyer convex lens

Basic principles of geometric optics

As said, geometric optics is a model that describes light propagation in terms of light rays. As long as the light beam propagates in an environment of the same density, nothing interesting happens. But if a substance or material of another density, such as water or glass gets in its way, the light beam may refract. For example, when a light ray enters at an angle from a lower-density medium (such as air) into a higher-density medium (such as glass) it slows down and changes direction. Similarly, light is refracted when it moves from an optically denser medium to an optically thinner medium. Light does not bend, only when it falls at right angles to the material.

light ray refraction

Before we investigate, what happens to the rays of light as they go through the smartphone screen magnifier, we need to clarify some of the concepts in geometric optics, that relate to lenses.

1) The main optical axis (OA) of the lens is a straight line passing through the two centers of curvature.

2) The focus is the point (F) in front (or behind) of the lens, where the rays parallel to the optical axis intersect.

3) The light ray passing through the center (0) of the lens does not change direction.

light rays and convex lens

Equipped with basic knowledge of geometric optics, we can draw ray diagrams and investigate what happens to the image on the smartphone screen if we place it at a certain distance from the magnifying glass.

Smartphone behind the magnification screen

Let's put our smartphone first in the point, which is located somewhere behind the focal point of the magnifying screen, and examine how the image on it changes. Drawing a ray diagram, we can see that the resulting image is larger than the original image. The image is upside-down and real. If the image is real, it means that it can be projected on the screen.

convex lens and real image

Let's try now, what happens to the image if we place the smartphone on the focal point of the magnifying glass. In this case, we can see from the diagram that light rays stay parallel and don't intersect. The image is infinitely large and is formed infinitely far away.

convex lens and image in the infinity

When we place the smartphone somewhere between the focal point and the center point of the magnifying glass, the image gets magnified. The image is right-side up and virtual. As we can see the image is formed on the same side of the magnifying glass as the original image itself. When the image is virtual, we can not project it onto a screen.

virtual image in the smartphone screen magnifier

So, for the image on the phone screen to be magnified, the smartphone must be placed somewhere inside the focal point behind the magnifying glass. There is a phone holder in this place behind the magnifying glass.

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