# Topic 4: Lenses and Vision

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Topic 4: Lenses and Vision
Unit 3: Light and Optics

I can... Explain how double concave and double convex lenses work.
Label the parts of the human eye compare the human eye to a camera

Types of Lenses Recall-- Concave means... Convex means...
Like a cave-- curved IN Convex means... Curved OUT

Double Concave Lens Thinner and flatter in the middle
What will happen to light as it passes through? Why?

Demo Time Let's see what happens to light as it passes through a double concave lens. Make a hypothesis: What actually happened? Why?

Double Concave Lens The light that goes through the thicker more curved areas bends more. The rays of light spread out (or diverge) Applications? Headlights, flashlight...

Double Convex Lens Thicker in the middle
What will happen to light as it passes through? Why?

Double Convex Lens The rays of light come together (or converge)
Applications? to create a bright spot i.e. Spot light.

Lenses and Images When the light rays converge, an image forms.
The light from the top is directed to the bottom the light from the bottom is directed to the top The image may be upside-down

The upside down image Images formed by a convex lens are upside down.

Mirrors vs. Lenses What’s the difference between mirrors and lenses?
Mirrors ______________ light Lenses _______________ light reflect refract

Recap: Concave means: Convex means: Which lens causes light to converge (come together)? What happens when light goes through a double concave lens? What lens will have an upside down image? curved in curved out Double concave It diverges (spreads out) A double convex lens

The Eye Is the lens in the eye concave or convex? It is CONVEX
**hint -- we want it to focus (converge) the light It is CONVEX It takes light rays from objects, refracts the rays, and focusses them on the retina.

Sight problems Near-sighted (can’t see objects from far away)
The eye is too long the image forms in front of the retina Can correct it with a concave lens to move the image back.

Sight problems Far sighted (can’t see objects that are close)
Eye is too short Image has not formed by the time it reaches the retina (it would form behind the retina) Correct vision with a convex lens to bend the rays to form on the retina

Your eye is like a camera
What do you think the function of each part is?

Focussing Camera If an object moves closer to the film, the lens needs to move farther from the film to keep it in focus. Eye Can't move the lens away from the retina... So The ciliary muscles change the shape of the lens if the object comes closer to you, the lens bulges in the middle Object stays in focus on the retina, and the eyeball doesn't have to stretch.

What is this adjusting of the lens called?
Accommodation What happens to your ability to accommodate as you age? Your lens stiffens and can't bend as easily The result? Reading glasses.

The eye can see a range of distances The shortest distance is called:
the near point for an adult it is around 25 cm. for a baby it is around 7 cm. The longest distance is called: the far point For an adult it is... infinity. You can see the stars after all.

Let there be light The camera Enough light needs to reach the film
The diaphragm controls the aperture (the opening) of a lens. The shutter can let in more light by staying open longer.

Let there be light The eye
The iris (the colored part) is like the diaphragm. The iris controls the size of the eye’s opening (or pupil). it opens up more when the light is dim.

Pupil Dilation The iris reflex is the when your pupil naturally changes sizes. What happens to your pupil if you walk into a dark movie theatre? Your pupil dilates or opens up to let in more light What happens to your pupil when you walk outside on a bright sunny day? Your pupil constricts or closes up to let in less light

Try it Turn to the person beside you. Label yourselves A and B
Person B, observe person A’s pupils. Person A, close and cover your eyes for 10 seconds. Open them and let Person B observe your pupils again. Switch roles.

Seeing the image Camera: image is focussed on the light-sensitive film
light energy --> chemical changes in film --> image is recorded

Seeing the image Eye: Cells in the retina detect light.
They send electrical impulses to the brain through the optic nerve Where the optic nerve connects to the retina, there are no light-sensing cells This is your blind spot.

Draw this picture Hold your page at arms length, cover your right eye with your hand Stare at the X while you move the page slowly towards yourself. The dot should disappear and then reappear as it moves onto your blind spot and then off again.

Case it How are the parts of a camera kept protected? A hard case.
How are the parts of an eye kept protected? the sclera or white part is the tough case that protects the eye. it is also connected to the muscles that control the eye. Filled with fluid to keep it from collapsing.

Liquid in the eye What is this liquid called? Humour
humour is an old word for “bodily fluids” People used to think your mood was controlled by the humour in your body. hence the saying “he is in good humour”.

Vitreous Humour the gel that fills the back part of the eye.
What is it’s function? It helps the eye keep it’s shape. Aqueous Humour the gel that is between the cornea and iris. It provides nourishment to the cornea and lens, and gives the eye it’s shape

Label the eye Label the: optic nerve, ciliary muscle, aqueous humour, lens, vitreous humour, pupil, iris, retina, cornea

Let's review 1. Show what happens to light when it passes through a concave lens. it diverges 2. Show what happens to light when it passes through a convex lens. It converges

3. Which part of the human eye corresponds with each camera part:
Eye Part Why Film Retina Detect light Diaphragm Iris Control the size of the opening where light passes through. Aperture Pupil The opening that light goes through Lens Lens and cornea Focus the light into an image Hard case Sclera Protect the parts inside