Optics is the study of light and the study of how we see. People have been studying and playing with light for millenia; highly polished stones over 8,000 years old acting as mirrors have been found in archaeological sites.

This story was created for the Google Expeditions project by Vida Systems, now available on Google Arts & Culture.

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Galileo Galilei, a 17th century scientist, was fascinated by the behavior of light and invented one of the world's first telescopes. He is also reported to be the first person to try and measure the speed of light.

Geometric optics

Geometric optics describes the behavior of light as it travels in the form of rays. It concentrates particularly on the behavior of light through lenses and media such as water. Rays of light generally travel in a straight line until they hit something.


Light rays appear to bend upon entering water, a phenomenon called refraction. Refraction occurs at the boundary of the 2 media: air and water. This occurs due to the light slowing down slightly as it changes from the air medium to water.  

Direction of bending

When light passes into a more dense material (in this case, water), it bends away from the surface between the 2 materials. The light entering the glass of water is called the incident ray, and the light traveling through the water is the refracted ray.


An angle of incidence is the angle that an incident ray makes perpendicular to the surface with which it makes contact. The angle of refraction is the angle made by a refracted ray perpendicular to the refracting surface.

Convex lenses

Lenses take advantage of refraction to allow people to see objects more clearly. A convex lens, a lens that curves outwards, like a magnifying glass, causes the light rays to converge to a single point of focus.

Concave lenses

Concave lenses, lenses that curve inwards, cause light rays to spread out. Like a peephole in a door, they make images look smaller than they are. The center of this type of lens is thinner than its edges.


We are able to see all sorts of objects around us because light reflects off of them. After a light source hits the surface of an object, it gets reflected in one of 2 ways: specular or diffuse. 

Diffuse reflection

Diffuse reflection occurs when light hits an object with a rough surface and is bounced back in a scattered way, in all different directions. Most everyday objects present as diffuse reflection because they have tiny imperfections on their surfaces.

Specular reflection

Specular reflection occurs on smooth surfaces, such as clear, still water, where all the rays of light reflect at the same angle. Each individual ray of light follows the law of reflection: the angle of reflection is equal to the angle of incidence. 


Light reflecting off objects, whether through diffuse or specular reflection, enters our eyes and travels to the retina at the back of our eyes. The retina converts these light rays into electrical signals for the brain to interpret.


In a vacuum, light travels in a straight line. Starting with Earth’s atmosphere, light that travels to Earth starts to interact with matter. Light behaves and travels differently when going through different material.

Light traveling through clear materials, such as glass and water, causes refraction — the bending of light rays.


Light travels slightly slower through water, as water is more dense than air. Of course, we don’t notice, as it normally travels at almost 186,000 miles per second, but slows down to 143,100 miles per second when it passes through water.


Light becomes bent and changes direction as it travels from the medium of air to water. Rays of light slow down once they enter the water, causing something like a slower moving traffic jam. This phenomenon is called refraction.


Due to refraction, fish and other objects within the water look higher, and closer to us, than they actually are. This happens because the light rays diffusing from the fish bend as they leave the water.


Many predators have a natural understanding of the concept of refraction. By using this ability that they were born with, they can compensate their aim when they go hunting for fish and other water–dwelling animals. 


Dispersion occurs because wavelengths of light travel at different rates of speed. This isn’t something that we’re usually able to notice. However, some materials will cause these wavelengths to separate, and that allows us to see rainbows.

White light

Light from the sun that travels through space to reach us on Earth is composed of 7 different colors that all travel at the same speed while in the vacuum. Together these colors combine to form “white light.”


A specially designed piece of glass called a prism splits this white light into its 7 wavelengths. Prisms have a triangular cross-section that causes the light to refract when it enters and exits the prism.


White light enters the prism as the ray of incidence. Due to the angles within the prism, each of the color wavelengths are refracted at slightly different angles. Red wavelength becomes the least bent, while the violet wavelength is bent the most.


Lenses need to be carefully designed in order to minimize the effect of dispersion. If the lens is designed incorrectly, it may focus different colors of light at different spots, impacting the functionality of the lens. 


The human eye has a limited amount of “screen space” available. It can only receive a certain amount of light information that the brain is able to process. 

Viewing a planet in detail with the naked eye is impossible because it’s so far away that it only takes up a tiny portion of available screen space. If our eyes were larger, then more information could be collected. 


The first telescope was invented in the early 1600s. We use these devices to view objects that are extremely far away in detail. Essentially a telescope acts as a larger “eye,” collecting more light than the human eye.


Reflector telescopes use mirrors to collect and reflect as much light as possible. The larger the size of the mirror, the more light it can collect. The giant Hubble telescope has a mirror over 8 feet in diameter. 


A traditional reflector telescope has a concave mirror at one end. The concave mirror collects light that is emitted from the distant stars and other celestial objects and then disperses the light into a wider area. 

Eyepiece lens

The light information collected by the concave mirror is reflected onto another mirror, which is reflected into an eyepiece lens. This lens has a  convex shape, magnifying the mirror’s image and taking up lots of eye screen space.


Lenses are specially designed pieces of glass that refract light in different ways. A lens can be used to see incredibly small things using a microscope, or to view things incredibly far away using a telescope. 

Many animals, including humans, have natural lenses located in their eyes. 


The human eye has a clear, flexible tissue that acts as a lens. Muscles around the eye pull on the lens, flattening it to see things far away. Pushing the tissue together to form a convex lens allows humans to see objects close up.

Camera lens

Camera lenses can use a series of both convex and concave–shaped glass to produce the desired image. The lenses direct light to film or, in the case of a digital camera, to a computer chip that can sense the light.

Image received

With both the human eye and a camera lens, the image is received in an upside–down position. This happens because the light is bent by the curvature of the lens. However, our brains interpret the image the correctly.

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