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Chapter 13: Problem 30

When you take pictures with a camera, the distance between the lens and the film has to be adjusted, depending on the distance at which you want to focus. This is done by moving the lens. If you want to change your focus so that you can take a picture of something farther away, which way do you have to move the lens? Explain using ray diagrams.

Short Answer

Expert verified
Move the lens closer to the film to focus on distant objects.

Step by step solution

01

Understand Lens Focusing

When a camera lens focuses on an object, it bends light rays to converge on the film or sensor. The lens-to-film distance changes based on the object's distance.
02

Determine Closer Focus Requirement

If an object is closer to the lens, the image forms farther from the lens, requiring increased lens-to-film distance to maintain focus.
03

Determine Farther Focus Requirement

For objects farther from the lens, the image formed will be closer to the lens, necessitating a shorter lens-to-film distance. This is because the light rays enter the lens closer to parallel when focusing on distant objects.
04

Move Lens for Farther Objects

To focus on distant objects, the lens must be moved closer to the film to reduce the lens-to-film distance due to the nearly parallel light rays.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Ray Diagrams
Ray diagrams are a handy tool in understanding how light behaves as it passes through a camera lens. When you take a picture, the lens helps to bend and direct light rays so that they converge onto the film or sensor to create a clear image. For every object you want to photograph, a corresponding set of rays travels through the lens. These diagrams typically show several rays emanating from a single point on the object, passing through the lens, and converging at a point where the image is formed on the film. Diagrams can reveal that:
  • When light rays from a distant object hit the camera lens, they are nearly parallel.
  • Conversely, light rays from a nearby object converge more sharply.
Through ray diagrams, you can see how the position and curvature of the lens affect where on the film or sensor the image will appear sharp.
Lens-to-Film Distance
The lens-to-film distance is crucial for focusing a camera correctly. It refers to the space between the lens and the film or image sensor inside the camera. This distance must be adjusted based on the distance of the object being photographed. When photographing something up close, you need to increase this distance, since the image forms further from the lens.
On the flip side, for faraway objects, the image forms closer to the lens, thus requiring a decrease in the lens-to-film distance.
  • Farther objects reduce the lens-to-film distance due to parallel rays.
  • Closer objects need a longer lens-to-film distance as they create divergent rays.
Understanding this concept helps photographers ensure their images are sharply focused.
Light Convergence
Light convergence refers to how light rays meet at a point after passing through a lens. When a camera lens captures light from an object, it gathers and bends the light rays so that they meet and form a clear image on the film or sensor. The angle at which light rays enter the lens determines how quickly they converge.
For objects that are far away, the rays are almost parallel when they enter the lens, meaning they converge closer to the lens. But for objects nearby, the rays diverge more when they hit the lens, requiring more distance to converge back into a focused image.
  • Near objects necessitate more space for ray convergence.
  • Distant objects demand less convergence space.
This concept is fundamental for adjusting camera focus and achieving sharp photographs.
Optics for Photography
Optics for photography involves the study of how lenses capture and manipulate light to produce images. A camera lens works by focusing the light reflected or emitted from objects onto the film or digital sensor. The goal is to create a sharp image with good clarity. There are a few key principles in optics relevant for photography:
  • Different focal lengths affect depth of field and the magnification of subjects.
  • Lenses with different apertures control how much light enters the camera.
Understanding these principles allows photographers to creatively control focus, lighting, and composition.
Whether working with film or digital, mastering optics is essential for capturing stunning photographs.

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Most popular questions from this chapter

The beam of a laser passes through a diffraction grating, fans out, and illuminates a wall that is perpendicular to the original beam, lying at a distance of \(2.0 \mathrm{~m}\) from the grating. The beam is produced by a helium-neon laser, and has a wavelength of \(694.3 \mathrm{~nm}\). The grating has 2000 lines per centimeter. (a) What is the distance on the wall between the central maximum and the maxima immediately to its right and left? (b) How much does your answer change when you use the small-angle approximations \(\theta \approx \sin \theta \approx \tan \theta\) ?

(a) A converging mirror with a focal length of \(20 \mathrm{~cm}\) is used to create an image, using an object at a distance of \(10 \mathrm{~cm}\). Is the image real, or is it virtual? (b) How about \(f=20 \mathrm{~cm}\) and \(d_{o}=30 \mathrm{~cm}\) ? (c) What if it was a diverging mirror with \(f=20 \mathrm{~cm}\) and \(d_{o}=10 \mathrm{~cm}\) ? (d) A diverging mirror with \(f=20 \mathrm{~cm}\) and \(d_{o}=30 \mathrm{~cm}\) ?

(a) A converging mirror is being used to create a virtual image. What is the range of possible magnifications? (b) Do the same for the other types of images that can be formed by curved mirrors (both converging and diverging).

Diamond has an index of refraction of \(2.42\), and part of the reason diamonds sparkle is that this encourages a light ray to undergo many total internal reflections before it emerges. (a) Calculate the critical angle at which total internal reflection occurs in diamond. (answer check available at lightandmatter.com) (b) Explain the interpretation of your result: Is it measured from the normal, or from the surface? Is it a minimum, or a maximum? How would the critical angle have been different for a substance such as glass or plastic, with a lower index of refraction?

Why would blue or violet light be the best for microscopy?
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