Diffraction and interference of light | Physics | Khan Academy

Summary

This video explores the fascinating phenomena of light diffraction and interference, explaining how light behaves as a wave when it encounters small obstacles. Using experiments like shining a laser through a hair strand, the instructor demonstrates how light bends, creating streaks and patterns. The video connects these effects to real-world examples, such as the streaks seen in Hubble Space Telescope images caused by structural components. The content explains how diffraction leads to spreading light, and interference creates alternating light and dark bands, offering an engaging exploration of wave behavior in light.

Takeaways

• 😀 Light behaves as a wave, and its interaction with obstacles leads to diffraction, which causes light to spread out.
• 😀 When light passes through small obstacles, like a hair strand, diffraction causes streaks of light rather than just a simple shadow.
• 😀 The phenomenon of diffraction happens when waves bend around an obstacle, not because the medium changes but due to the obstacle's presence.
• 😀 Diffraction is different from refraction, which occurs when waves change medium and speed, whereas diffraction occurs due to obstacles.
• 😀 In daily life, large objects that are much bigger than the wavelength of light don't show noticeable diffraction effects, which is why we usually don't see light bending around obstacles.
• 😀 When the obstacle's size is comparable to the wavelength of light, like a hair strand, diffraction becomes noticeable and causes visible streaks.
• 😀 A vertical hair strand in front of a light source creates horizontal streaks on a screen, while a horizontal strand creates vertical streaks.
• 😀 The crosshairs in the Hubble Space Telescope's mirrors create diffraction patterns, resulting in streaks appearing in images.
• 😀 In diffraction, constructive interference (peaks on peaks and troughs on troughs) creates bright spots, while destructive interference (peaks on troughs) creates dark spots.
• 😀 As you move farther from the source of the light, diffraction patterns spread out, causing the intensity of light to decrease and the pattern to become more pronounced.

Q & A

What causes the streaks seen in Hubble Space Telescope images?

The streaks in Hubble Space Telescope images are caused by diffraction, which occurs when light encounters obstacles such as the struts that hold the secondary mirror in place. These struts create patterns similar to those seen in a mosquito net's cross-hairs, leading to the streaks.

Why do we see streaks in photos taken on Earth, such as the one from the balcony?

The streaks seen in photos taken on Earth, such as the one from the balcony, are due to diffraction. When light interacts with small obstacles, like streetlights or a hair strand, it bends and spreads out, creating streaks in the image.

What is the phenomenon that explains how waves bend around obstacles?

The phenomenon that explains how waves bend around obstacles is called diffraction. Diffraction occurs when waves encounter an obstacle or aperture, causing them to spread out and bend around the obstacle.

How does diffraction differ from refraction?

Diffraction involves waves bending around obstacles without a change in the medium, while refraction is the bending of waves due to a change in speed when they pass from one medium to another.

Why don't we usually notice diffraction effects in our daily life?

We don't usually notice diffraction effects in daily life because most objects are much larger than the wavelength of light. The effects of diffraction become noticeable only when the object is small compared to the wavelength of light, such as with a hair strand.

What role does the size of the obstacle play in diffraction?

The size of the obstacle plays a significant role in diffraction. If the obstacle is comparable in size to the wavelength of light, diffraction effects are noticeable. However, if the object is much larger than the wavelength, diffraction effects are negligible.

Why do we see dark spaces between the streaks in diffraction patterns?

The dark spaces between the streaks in diffraction patterns are caused by destructive interference. When the peaks of one wave coincide with the troughs of another wave, they cancel each other out, resulting in regions of low or no light intensity.

How does the laser experiment with a hair strand illustrate diffraction?

In the laser experiment with a hair strand, the laser light diffracts around the hair, creating streaks on the wall. The diffraction pattern becomes more pronounced as the distance from the obstacle increases, showing how light spreads out after passing around the hair.

What happens when light encounters an obstacle larger than its wavelength?

When light encounters an obstacle much larger than its wavelength, the diffraction effects are minimal. The shadow cast by the object will be nearly the same size as the object itself, and diffraction has little impact on the light pattern.

What is constructive interference in the context of diffraction?

Constructive interference occurs when the crests of one wave align with the crests of another wave, or the troughs align with the troughs. This results in an increase in light intensity at those points, creating bright spots in the diffraction pattern.