# Waves- Lenses and Diffraction

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### Distinguish between a converging lens and a diverging lens.

A converging lens is thicker in the middle and rays of light that are initially parallel are made to converge. A diverging lens is thinner in the middle and the rays of light are made to diverge.

### Distinguish between a virtual image and a real image.

A virtual image cannot be seen on a screen at the image distance because no light is actually directed to the image position. The rays that reach your eye behave as if they came from the image position, however. A real image is formed when light from the object does converge and can be focused on a screen.

### There are three convenient rays commonly used in ray diagrams to estimate the position of an image. Describe these three rays in terms of their orientation with respect to the principal axis and focus points.

The first ray runs parallel to the principal axis, the second ray runs directly through the center of the lens, and the third ray travels through the focal point and then parallel to the principal axis.

### Which instrument- a telescope, a compound microscope, or a camera- is most similar to the eye?

The camera is most similar to the eye.

### Distinguish between farsighted and nearsighted vision.

The eyes of a farsighted person form images behind the retina because the eyeball is too short. Farsighted people have to hold things more than 25 cm away. A nearsighted person can see nearby objects clearly, but does not see distant objects clearly because they are focused to near the lens, in front of the retina. The eyeball is too long.

### What is astigmatism, and how can it be corrected?

Astigmatism of the eye is a defect that results when the cornea is curved more in one direction than the other. Because of this defect the eye does not form sharp images. The remedy is cylindrical corrective lenses that have more curvature in one direction than another.

### (a) What condition must exist for a converging lens to produce a virtual image? (b) What condition must exist for a diverging lens to produce a real image?

a) A converging lens will produce a virtual image only when the object is between the focal point and the lens.
b) A diverging lens will never produce a real image.

### How could you demonstrate that an image was indeed a real image?

To demonstrate than an image was indeed real, you could show that it appears on a screen.

### Why do you have to put slides into a slide projector upside down?

Because the images that are produced will be upside down if you do not, so by putting it upside down the image will appear right side up.

### Would telescopes and microscopes magnify if light had the same speed in glass as in air? Explain.

If light had the same speed in glass as in air, there would be no refraction, and therefore no focusing.

### a. Waves spread out when they pass through an opening. Does spreading become more or less pronounced for narrower openings? b. What is the spreading called?

a) If you narrow the opening waves will spread out more, producing a more pronounced shadow.
b) This spreading is called diffraction.

### Does diffraction aid or hinder the viewing of images in a microscope?

Diffraction hinders the viewing of images in a microscope.

### Does wave interference occur for waves in general, or only for light waves? Give examples to support your answer.

Wave interference occurs for waves in general. Wave interference causes beat frequencies in sounds, as well as disturbances in ocean waves.

### What is the cause of the fringes of light in Young's experiment?

The bright fringes of light resulted from light waves from both holes arriving crest to crest (constructive interference-more light)

### What is a diffraction grating?

A diffraction grating is a multitude of closely spaced parallel slits.

### What is iridescence, and to what phenomenon is it related?

Iridescence is the colors produced by the interference of light waves of mixed frequencies in thin films. It is related to the phenomenon of interference.

### If a soap bubble is thick enough to cancel yellow by interference, what color will it appear if illuminated by white light?

The soap bubble will appear blue when illuminated by white light.

### How does light from a laser differ from light from an ordinary lamp?

Light emitted by a common lamp is incoherent, meaning it has many phases of vibration as well as many frequencies. A beam of light produced by a laser is coherent, meaning it has the same frequency, phase, and direction.

### Why do radio waves diffract around buildings but light waves do not?

Because radio waves have longer wavelengths, they are able to diffract around buildings while light waves are not.

### A far-sighted eye whose comfortable reading distance is 80 cm is to be supplied a corrective lens to enable reading at a normal distance of 40 cm. So the object distance is 80 cm and the image distance is 40 cm. What is the focal length of the corrective lens?

o = object distance i = image distance f = focal length

1/o + 1/I = 1/f

1/80 + 1/40 = 1/f

f = 26.7

26.7 cm

red and blue

refraction

virtual

### What is diffraction?

bending of a wave around a barrier

### A diverging lens will produce an image that is ___________________________.

virtual and right-side up

47⁰

### When monochromatic light waves interfere due to diffraction, ____________________.

light fringes indicate constructive interference

### List the following as they appear on the electromagnetic spectrum: x-rays visible light radio waves You should also indicate the direction of increasing frequency and the direction of increasing wavelength.

visible light
x-rays (highest frequency/shortest wavelength)

### Explain why the atomic spectra is considered the "fingerprint" of an element.

The atomic spectra is unique for each element.

### capacitor

a device used to store charge in a circuit

### electric field

a force field that fills the space around every electric charge or group of charges

### electric potential

electrical potential energy per coulomb (J/C) at a location in an electric field; measured in volts and often called voltage

### electrical potential energy

energy a charge has due to its location in an electric field

Example: