How & Why Does Light Refract Through A Glass Lens?

As an Amazon Associate we earn from qualifying purchases made on our website.

When light encounters an object, it can either be reflected, absorbed, or transmitted. If the object is transparent, like glass, the light is transmitted through it—but not without bending first! And if it’s a glass lens, this bending is even more interesting. But why does light refract when it encounters the glass in a lens?

When a ray of light hits the surface of a lens, it enters into a new material (glass) denser than air. The change reduces the speed of the photons, and as a result, the light bends. The degree of bending depends on the difference in the densities of the two mediums and the angle of the light ray. 

Reflection is what allows us to see objects. When light waves bounce off an object and into our eyes, we perceive that object’s color, shape, and size. But when light refracts through a lens, we see things in a whole new way. From microscopes to eyeglasses to telescopes, refraction through lenses has given us a whole new perspective on the world. Let’s learn more about why light refracts in glass and how lenses work.

Why Does Glass Refract?

A glass house near green

To understand why glass refracts, we need to take a step back and understand how light reacts when it encounters different materials. You can use a tiny prism of glass (on Amazon) to see this effect, but for now let’s just unpack the details.

Light is made up of tiny packets of energy called photons. In a vacuum, photons travel in a straight line at a constant speed. But when they encounter matter, air, liquid, or gas, they interact with the atoms and molecules of that medium. This interaction happens in one of three ways:

  • Reflection: The photons bounce off the surface of the object, as in the case of a mirror.
  • Absorption: The photons are absorbed by the material, as in the case of a black object.
  • Transmission: The photons pass through the material, as in the case of a pane of glass.

The photons don’t make it through the material with reflection and absorption. But with the transmission, they do, and it’s a unique property of transparent materials like glass.

However, just because the photons make it through doesn’t mean they don’t interact with the atoms in the glass. They do—they slow down and change direction. This change in direction is called refraction. 

The ability to refract light is measured as the refractive index. The higher the refractive index, the greater the bending of light. And glass has a high refractive index of 1.5.

To put it in perspective, the refractive index of air is about 1.0, Water is 1.3, and Diamond is 2.4. So glass bends light more than air but not as much as a diamond.

How Does Light Refract Through Glass?

As we learned, light travels in a straight line until it encounters an object. When the object is transparent, like glass, the photons pass through it, but at the point where they enter from one medium (air) into another (glass), they change speed. This change in speed causes the photons to bend or refract.

Let’s do a small experiment. Find a glass cube or a clear drinking glass and place it upside down on a table. Flash a colored laser pointer at the side of the glass (Turn off the room lights so you can see the laser beam more easily).

You should see the beam of light bend as it enters the glass and continues straight through it. And then bend again as it exits the glass and enters the air, but this time the direction is reversed.

So the main event here is the change in speed. The photons slow down as they enter the glass and then speed up again as they leave. Since the medium of entry and exit remains the same, the angle at which the light enters also equals the angle at which it leaves. The only difference is the horizontal shift, which you can observe in the experiment.

How Does a Glass Lens Refract Light?

Glass wall

A glass lens is simply a piece of curved glass. The curve can be convex (bulging outwards) or concave (curving inwards). The principle of refraction in lenses is similar to that of parallel-sided glass—the photons slow down, and the light bends. However, the curvature of the lens affects the degree to which the light bends.

When a beam of light passes through a convex lens, the photons are refracted inwards towards the center of the lens, or the beam of light converges. For example, our eyes have convex lenses, which focus the light on our retina to form an image.

On the other hand, when a beam of light passes through a concave lens, the photons are refracted outwards, and the beam of light diverges. This type of lens is used in projectors and torches.

By changing the degree of curvature, the thickness of the lens, and the type of glass, we can control how much and where the light bends. This is how lenses are used in optical instruments like cameras, microscopes, and telescopes.

Leave a Comment