Objectives
After working through this chapter, you should be able to:
Explain how the human eye focuses distant light on the retina
Explain the different contributions of refractive index and curvature on an image
Introduction
In this book we’ve learned a lot about light and how it interacts with both flat (plane) and curved surfaces. We’ve also learned that when light moves from a material of one refractive index to another, it will undergo refraction (a change in direction). We’ve also touched on the idea that the human eye (see chapter 5 for revision on this) is able to focus light on the back of the eye through two key characteristics:
- 1.
The cornea (front of the eye) is a different refractive index to that of air (cornea 1.376; air 1.00), which allows refraction to take place. Importantly the fluid inside the eye that sits behind the cornea (the aqueous ) is a similar refractive index to the cornea.
- 2.
The cornea is a convex, curved shape , which means it has dioptric power . This allows the cornea to add convergence to the incoming light ( Fig. 22.1 ).
• Fig. 22.1
An illustration of a cross-section of a human eye with light (blue lines) entering through the cornea (labelled) and being focused onto the fovea in the retina (labelled). The refractive index of air (n = 1.00) and the refractive index of the cornea (n′ = 1.376) and aqueous (n′ = 1.336) are labelled for information.
This is important because it shows us that the basic focusing of distant light onto the back of the eye has nothing to do with any muscles (which is a common misconception) and can therefore not be ‘trained’. Instead, if a refractive error is present and light focuses too early (myopia) or too late (hyperopia), it can only be corrected by altering the vergence of the light before it enters the eye (glasses or contact lenses), or by changing the shape of the cornea (refractive surgery). To prove this, this chapter will explain how we can make a mock (relatively huge) cornea at home using common household items.
The experiment
The goal of this experiment is to show that the refractive index change alone would not be enough for the eye to focus light over such a short distance (corresponding to the length of the eyeball); instead we’re going to prove that it’s the curvature of the cornea that adds all the power. To that end we need to produce a convex surface that’s a different refractive index to air and see how the image compares to a flat surface that’s a different refractive index to air. Please note, however, that there will be quite a size difference between our homemade ‘cornea’ and a real cornea, so the radius of curvature is not equivalent and so our homemade cornea will not have a power equivalent to a real cornea. Instead, it will serve to demonstrate the basic principles.
Before starting the experiment, please make sure you have all the equipment you need (and a mobile device to record your results!).
Equipment required
- •
An empty, clear plastic 2 L fizzy drink bottle – note the shape of the bottle is quite important so try to find one that looks like the one in Fig. 22.2 .
• Fig. 22.2
A diagram of all the equipment needed for this experiment, including: a pen, scissors, water, a clear flat plastic tub, a 2 L empty plastic bottle and a doodle or some writing on a piece of paper. Please note that the 2 L bottle needs to be clear plastic and needs to be this shape (round ‘shoulders’ near the top).
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A pair of scissors
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A felt-tip pen or marker of some kind
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An empty, clear plastic flat tub; this could be a food storage container or an empty packet of sliced cheese – anything that’s flat, plastic and completely clear.
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Water
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An object that can be laid flat on the table, which we will look at through our homemade cornea; this could be a drawing on a piece of paper (like in Fig. 22.2 ) or possibly something written down, but please don’t use anything electrical, valuable or important, because there is a small risk of spilling the water onto the object.
- •
Optional (but advised): a towel or kitchen roll to mop up water spillages
Method
- 1.
Using the pen, draw a circle shape on the top ‘shoulder’ area of the 2 L bottle (like shown in Fig. 22.3 ). Make sure the circle is a reasonable size (at least 5 cm in diameter). It does not have to be perfectly spherical.
