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Projectile motion - 1 VCE Physics.com & vision

• The human eye • Near point • power • Short sightedness () • Long sightedness (Hypermetropia) • Using

Optics & vision - 2 VCE Physics.com The human eye

• The & lens of the eye act to focus light into a real image on the retina. • The image distance is the length of the eye-ball: about 2.5 cm. • Changes to focus are the result of reshaping the lens by the ciliary muscles.

Optics & vision - 3 VCE Physics.com The human eye

• Real images are projected through the lens to the retina. • The image distance is the length of the eye-ball: about 2.5 cm. • is change in the to view objects at different distances. • Over time, accommodation reduces as the ciliary muscles weaken. Object

Image

v = 2.5 cm

Optics & vision - 4 VCE Physics.com Near point

• The near point of vision is the shortest distance at which a clear focused image can be seen. • 25 cm is the medical text book definition but.... • The near point gets further with age: • ~7 cm at age 10, ~16 cm at age 40, ~1 at age 60. • Most 60-year-olds must use corrective lenses to read books or magazines at a comfortable distance - “reading ”.

Optics & vision - 5 VCE Physics.com Lens power

• Focusing on a closer object requires more convergence of light. • This requires a more powerful lens. • 1 dioptre = the power of a lens with a 1m focal length. • The powers of two combined lenses add together. • Converging lenses have positive power, diverging lenses have negative power.

1 Lens power = Focal length (m)

dioptre (/m)

Optics & vision - 6 VCE Physics.com Lens power

• Focusing on a closer object requires more convergence of light

Focusing on a distant object Focusing on a near object v = 2.5 cm = 0.025 m, u =∞ v = 2.5 cm, u = 25 cm

1 1 1 1 1 1 = + = + f 0.25m 0.025m f u v 1 100 1000 1 1 = + = 0+ f 25 25 f v 25 f =v = 0.025m f = =2.3 cm 1100 1 1 1100 P = = = 40D P = = 44D f 0.025 m 25 Optics & vision - 7 VCE Physics.com Short sightedness (Myopia)

• Myopia is an inability to focus on distant objects. • The cornea & lens causes too much convergence of light for the length of the eyeball. • The image is focused in front of the retina. • Diverging lenses (negative power) are prescribed to reduce the affects.

Optics & vision - 8 VCE Physics.com Long sightedness (Hypermetropia)

• Hypermetropia is an inability to focus on close objects. • The cornea & lens don’t give enough convergence of light for the length of the eyeball. • The image is focused behind the retina. • Converging lenses (positive power) are prescribed to reduce the affects.

Optics & vision - 9 VCE Physics.com Using lenses

• eg. A 60 year old person with a near point of 1.0m. • What strength reading glasses should be used to bring the near point to 25 cm? v = 0.025 m, u = 1 m v = 2.5 cm, u = 25 cm 1 1 1 1 1 1 = + = + f 1m 0.025m f 0.25m 0.025m 1 25 1000 1 100 1000 P = = + P = = + f 25 25 f 25 25 1025 = = 1100 P 41D P = = 44D 25 25

The reading glasses must be +3.

Optics & vision - 10 VCE Physics.com Using lenses

Optics & vision - 11 VCE Physics.com Projectile motion - 12 VCE Physics.com