physics chapter 14 and 15

8/2/2019 Physics Chapter 14 and 15

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• REFRACTION

• Light rays reflect and refract.

• If the light slows down, it bends toward the normal line ( glass < air ).

– Angles are measured with the normal line.

– Light rays are reversible.

LAW OF REFRAC

• c = 3 108 m/s

• v is always less than c, so n >1 for all media.

– nair = 1.000293

• n is dimensionless.

• n is a measure of the optical density of a material.



• Angles must be measured with the normal.

LENSES-REFRACTION

A lens is a transparent object that converges or diverges light by refraction.

• A converging lens is thicker at the middle.

• A diverging lens is thinner at the middle.

• Light actually bends at each surface. However, for thin lenses, we can show light bending only

once at the center of the lens.

• Focal length ( f ) is the distance from the focal point (F ) to the center of the lens.

Refraction-Sign Conventions

• p is positive if the object is in front of the lens.



• q is positive if the image is behind the lens (real and inverted).

• q is negative if the image is in front of the lens (virtual and upright).

• f is positive for converging lenses and negative for diverging lenses.

• h and h’ are positive if upright and negative if inverted.

The Eye and Corrective Lenses-REfractiom

• Light is refracted at both the cornea (outer surface) and the lens.

– When functioning properly, the converging lens can adjust so that the image is focused

on the retina.

• Muscles adjust the thickness of the lens.

• Many people are nearsighted (myopia) and can’t see distant objects clearly.

• Older people are often farsighted (hyperopia) and can’t see nearby objects.

– The lens becomes inflexible with age and can’t be made thicker to focus on nearby

objects.

Nearsightedness

• The image forms in front of the retina, possibly because the retina is too long.

• What type of lens is needed in front of the eye to correct the problem, converging or diverging?

Explain your reasoning.

– Answer: a diverging lensFarsighted

• The image forms behind the retina, possibly because the lens is inflexible.

• What type of lens is needed in front of the eye to correct the problem, converging or diverging?

Explain your reasoning.

– Answer: a converging lens

Combining Lenses

• Microscopes and refracting telescopes use two lenses.

– The objective lens forms a real image that is located inside the focal point of the

eyepiece.

– The eyepiece magnifies the first image, creating a large virtual image.

• Total internal reflection occurs if the angle in the denser medium is too great.

– Light can’t emerge so it is reflected back internally.

– Occurs if the angle is greater than the critical angle ( c).

• Used in fiber optics, right angle prisms, and diamond cutting.



• c occurs when the angle in the less dense medium is 90°.

– At the critical angle, the emerging ray travels along the surface.

– At greater angles, the rays are totally internally reflected.

• Mirages are caused by the refraction of light as it strikes the

hot air near the earth’s surface.

– This phenomena can be observed when driving on blacktop roads on hot summer days.

• Inverted cars can be seen approaching, with the actual cars up above them.

Dispersion

• Refraction or n depends on the wavelength. – Longer wavelengths refract less.

• Prisms disperse the light into a spectrum.

• Chromatic aberration is a lens problem where different colors focus at different points.

– Can lead to imperfect images for cameras with less expensive lenses.

Refraction: Why does the lawnmower turn when it strikes the grass?

– The right wheel slows down before the left one.

– Light waves behave in the same way.

• Refraction is the bending (change in direction) of light when it travels from one medium into

another. Caused by a change in speed

physics chapter 14 and 15

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