How Does a Magnifying Glass Work?

Magnifying glass's working principle

Magnifying glasses are used to enlarge any object without distortion. But how does it work?

Luckily, the working principle of this popular optical instrument is not that complex to understand. The convex lens in the magnifying glass uses the principle of refraction to magnify. The “optical lens” bends the light rays entering its surface at a right angle. When you look through the magnifying glass, the light rays converge at your eye so that they form an enlarged but undistorted image of what is on the other side.

Let’s briefly discuss the working principle of a magnifier in this article. 

Refraction of Light

Light travels in a straight line in any medium. When it enters from one medium to another, it bends or changes its direction at the boundary. This phenomenon is called the refraction of light. Magnifying glasses work based on this principle.

As an electromagnetic wave, light has a traveling speed. The speed is fixed for any medium. It is maximum for free space (without air or any other particle) at 186000 miles per second.

The speed is directly related to the optical density of the medium in which the light is traveling. Optical density is not the same thing as physical density. Instead, it is related to the tendency of absorption of electromagnetic energy traveling through the medium. As the medium becomes optically denser, the velocity of light in that medium becomes slower.

The change in velocity between mediums makes the light bend its traveling path and causes refraction.

Demonstration of bending of light as it travels from one medium to another. This phenomenon is also called the refraction of light.

So, how do we measure refraction? It is measured by an angle between the light’s path in the medium it enters and the normal drawn to the surface between mediums.

Illustration showing incident angle, refraction angle, and the normal to the surface of air and glass.

Refraction also depends on the incident angle. It is the angle that the light’s path makes with the normal to the surface of the traveling medium. The greater the incident angle, the more the bending of the traveling path is noticeable.

An important parameter related to the optical density of any medium is its refractive index. We measure the refractive index of a medium from the ratio of the light velocity in free space to the velocity in the medium of interest. 

As light travels slower in any medium other than the vacuum, the refractive index for any medium becomes higher than 1. The higher the value of this parameter, the slower the light will be.   

We can not conclude our discussion on the refraction of light without spending some time on Snell’s law. Dutch astronomer and mathematician Willebrord Snellius found the relationship between the refractive index of two mediums in the direction of light’s travel and the incident and refraction angle. 

In a magnifying glass, refraction occurs twice. The lens in the magnifier is generally made of glass. When we see an object through it, the light rays change the medium twice in its traveling path towards our eyes. 

  1. The first change of medium occurs when light enters into the glass of the magnifier’s lens from the air.
  2. The second change occurs when light enters into the air medium again from the glass. 

Now, air and glass have different refractive indexes. As a result, the velocity changes each time the medium changes, causing the light to change its direction of travel at both lens surfaces. 

In a magnifying glass, refraction occurs twice. As shown in the figure, the light rays change the medium twice in their traveling path through the lens.
Photo credit: Wikipedia

Illusion of Lens

The lens used in a magnifier is bi-convex. The lens is curved outside on both sides and thicker in the center than the edges. Each side of the lens can be considered as a circumference of a circle. We can consider two such circles, one for each side of the lens. The centers of these imaginary circles are called the focal points.

As light rays pass through the convex lens of the magnifier, they bend. The refracted rays come out of the lens and converge to the focal point. That is why the convex lens is also called a converging lens. 

To the human eye, the convergence of light creates an illusion. We don’t notice light bending as the rays enter our eyes. But our brains make us think that rays are coming to our eyes following a straight path. 

We think light is coming from a larger object located behind the actual one. The object our eyes see is an undistorted magnified version of the original. 

Illustration of magnification of object with a magnifying glass

To create this illusion of magnification, the distance of the object from the lens is crucial. Depending on the placement of the original object with respect to the focal length of the magnifier, we see images of different sizes of the same object.

Image Formation 

Light rays reflected from the object’s surface will pass through the lens and enter our eyes. We will see an image of the object. Depending on the distance of the object, we will see different images. Some of them are straight, and some are inverted. Some are magnified, whereas some images are a miniature version of the original object.  

If the object is placed inside the focal point, the image formed by the magnifier will be extended and undistorted. We will see the magnified image located behind the original object.

If the object under inspection is just at the focal point, the lens will produce a distorted image. We can not view the image. So, placing the object right at the focal point of the far side is useless.

Suppose the object is placed outside the focal length but within a distance less than twice the focal length. In that case, we will see an inverted image of the original. The image will be magnified and undistorted. We will feel like the image is in front of the object we are observing. 

Object placed outside focal point of magnifying glass

Now, let’s consider the object is placed just at twice the distance of the focal point. It will create an inverted image of the same size as the actual object. 

Magnification when object is placed at twice the focal length from the lens

Placing the object further away will create a smaller and inverted version of it. The magnifier will be of no use in those cases.

Other Factors Influencing Magnification

Lenses with the focal points closer to them will give higher magnification for smaller objects. To bring the focal points closer, the surface of the lens becomes more spherical, and the region near the center becomes thicker. 

The size of the lens is also an essential factor to consider. Larger lenses will be able to accumulate more reflected light rays coming from the object under inspection. Viewers will be more comfortable magnifying things with it.

Conclusion

In this article, I have tried to explain how magnifying glass works as concisely as possible. The light reflected from the object we inspect passes through the magnifying glass and bends. This bending of rays, also known as refraction, makes an illusion in our eyes, and we see a magnified object. 

Depending on the distance between the object and our magnifying glass, magnification varies. Suppose we hold the magnifying glass far from the object. In that case, we can even observe a minification of the original object. 

This minification is done when the object is more than twice the focal length away from the magnifier. The size of the glass and the spherical shape of the lens are also important factors for the good magnification of miniature objects. Hope you will now understand the working principle of the magnifying glass. 

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