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Ball lenses are glass spheres which can be used for special effects in photography - Leaving Cert Physics - Question d - 2021
Question d
Ball lenses are glass spheres which can be used for special effects in photography. Light travels at a different speed in air and in glass. (i) The photograph show... show full transcript
Worked Solution & Example Answer:Ball lenses are glass spheres which can be used for special effects in photography - Leaving Cert Physics - Question d - 2021
Step 1
(i) The photograph shows the inverted image of a mountain formed in a ball lens. Draw a ray diagram to show how an inverted image is formed in a lens. Is the image real or virtual?
Answer
To illustrate how an inverted image is formed in a ball lens, we can draw a ray diagram.
- Begin by sketching a converging lens (ball lens) in the center.
- Draw the object (mountain) beyond the focal point (f) of the lens.
- Draw two rays originating from the top of the object:
- One ray parallel to the principal axis, which refracts through the focus on the opposite side.
- Another ray passing through the center of the lens, which continues in a straight line.
- The point where the rays meet on the opposite side of the lens indicates the position of the inverted image.
The image is real because it is formed on the opposite side of the lens from the object.
Step 2
(ii) What is meant by critical angle?
Answer
The critical angle is defined as the angle of incidence in a denser medium, beyond which light cannot pass into a less dense medium but is instead reflected back. It is the angle at which the angle of refraction is 90°. The critical angle can be calculated using Snell's law.
Step 3
(iii) Calculate the speed of light in the ball lens.
Answer
To calculate the speed of light in the ball lens, we can use the formula derived from the refractive index:
where:
- n is the refractive index,
- c is the speed of light in a vacuum (approximately m/s),
- v is the speed of light in the medium.
Given the critical angle is 41.4°,
- Calculate the refractive index:
- Using this to find v:
Step 4
(iv) Explain why white light is dispersed as it passes through the ball lens.
Answer
White light is composed of various colors, each having different wavelengths. When white light passes through a prism, or in this case, a ball lens, each color refracts at a different angle due to their differing speeds in the glass. This phenomenon is known as dispersion. The glass has different refractive indices for different colors, leading to the separation of light into its constituent colors.