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Questions and Answers
Define 1 dioptre of power of a lens.
Define 1 dioptre of power of a lens.
1 dioptre is the power of a lens whose focal length is 1 metre.
At what distance should the needle be placed in front of the convex lens if the image is equal to the size of the object?
At what distance should the needle be placed in front of the convex lens if the image is equal to the size of the object?
50 cm
Find the power of a concave lens of focal length 2 m.
Find the power of a concave lens of focal length 2 m.
-0.5 D
Which one of the following materials cannot be used to make a lens? (Select all that apply)
Which one of the following materials cannot be used to make a lens? (Select all that apply)
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Where should the object be placed to observe a virtual image larger than the object in a concave mirror? (Select all that apply)
Where should the object be placed to observe a virtual image larger than the object in a concave mirror? (Select all that apply)
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Where should an object be placed in front of a convex lens to get a real image the size of the object? (Select all that apply)
Where should an object be placed in front of a convex lens to get a real image the size of the object? (Select all that apply)
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A spherical mirror and a thin spherical lens have each a focal length of -15 cm. What are they likely to be? (Select all that apply)
A spherical mirror and a thin spherical lens have each a focal length of -15 cm. What are they likely to be? (Select all that apply)
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No matter how far you stand from a mirror, your image appears erect. What type of mirror is likely to be? (Select all that apply)
No matter how far you stand from a mirror, your image appears erect. What type of mirror is likely to be? (Select all that apply)
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Which of the following lenses would you prefer to use while reading small letters found in a dictionary? (Select all that apply)
Which of the following lenses would you prefer to use while reading small letters found in a dictionary? (Select all that apply)
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Find the range of distance of the object from the concave mirror to obtain an erect image.
Find the range of distance of the object from the concave mirror to obtain an erect image.
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Name the type of mirror used in headlights of a car.
Name the type of mirror used in headlights of a car.
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What happens when half of a convex lens is covered with black paper?
What happens when half of a convex lens is covered with black paper?
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Find the position, size, and nature of the image formed when an object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm.
Find the position, size, and nature of the image formed when an object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm.
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How far is the object placed from a concave lens of focal length 15 cm when it forms an image 10 cm from the lens?
How far is the object placed from a concave lens of focal length 15 cm when it forms an image 10 cm from the lens?
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Find the position and nature of the image when an object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm.
Find the position and nature of the image when an object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm.
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What does a magnification of +1 produced by a plane mirror mean?
What does a magnification of +1 produced by a plane mirror mean?
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Find the position of the image, its nature, and size when an object of length 5.0 cm is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm.
Find the position of the image, its nature, and size when an object of length 5.0 cm is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm.
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How far from a concave mirror should a screen be placed to get a sharp-focused image of an object of size 7.0 cm located 27 cm in front of the mirror of focal length 18 cm?
How far from a concave mirror should a screen be placed to get a sharp-focused image of an object of size 7.0 cm located 27 cm in front of the mirror of focal length 18 cm?
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Find the focal length of a lens of power -2.0 D. What type of lens is this?
Find the focal length of a lens of power -2.0 D. What type of lens is this?
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Find the focal length of a corrective lens prescribed by a doctor with power +1.5 D. Is it diverging or converging?
Find the focal length of a corrective lens prescribed by a doctor with power +1.5 D. Is it diverging or converging?
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Study Notes
Light – Reflection and Refraction
- Visibility of objects depends on light; darkness prevents sight.
- Objects reflect light, which is detected by our eyes, enabling visibility.
- Light travels in straight lines, evidenced by sharp shadows of opaque objects.
- Diffraction occurs when light bends around small objects, indicating light behaves as both a wave and a particle.
- This chapter focuses on reflection and refraction of light, particularly using the principles of ray optics.
Reflection of Light
- Reflection occurs primarily on polished surfaces, such as mirrors.
- Laws of reflection:
- The angle of incidence equals the angle of reflection.
- The incident ray, the normal at the point of incidence, and the reflected ray lie in the same plane.
- Plane mirror images are always virtual, erect, the same size as the object, and laterally inverted.
Spherical Mirrors
- Spherical mirrors can be concave (inward-curving) or convex (outward-curving).
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Terminology:
- The pole (P) is the point on the mirror's surface.
- The center of curvature (C) is outside the mirror's reflecting surface.
- The radius of curvature (R) measures the sphere radius; for small apertures, R = 2f (where f is the focal length).
- The principal axis is a straight line through P and C, normal at P.
Concave Mirrors
- Concave mirrors converge light rays to a single point (focus).
- The focus (F) is located on the principal axis.
- Focal length (f) is the distance from P to F.
- A concave mirror can ignite paper by concentrating sunlight, demonstrating the focus.
Image Formation by Spherical Mirrors
- Image characteristics depend on the object's position relative to P, F, and C:
- Object at infinity: image at F, highly diminished, real, inverted.
- Object beyond C: image between F and C, diminished, real, inverted.
- Object at C: image at C, same size, real, inverted.
- Object between F and C: image beyond C, enlarged, real, inverted.
- Object at F: image at infinity, highly enlarged, real, inverted.
- Object between P and F: image behind the mirror, enlarged, virtual, erect.
Ray Diagrams and Image Representation
- Ray diagrams illustrate image formation by showing light ray paths:
- A parallel ray to the principal axis reflects through F (concave) or appears to diverge from F (convex).
- A ray through F reflects parallel to the principal axis.
- A ray through C reflects back on the same path.
- An obliquely incident ray reflects obeying the laws of reflection.
Summary of Image Formation
- The nature, position, and size of images formed by concave mirrors depend on object placement.
- Ray diagrams should be created for varying object positions to visualize image characteristics effectively.### Concave Mirrors
- Commonly used in torches, searchlights, and vehicle headlights for powerful parallel beams of light.
- Shaving mirrors provide an enlarged view of the face for precision.
- Dentists utilize concave mirrors to examine larger images of patients' teeth.
- Solar furnaces employ large concave mirrors to focus sunlight for heat generation.
Convex Mirrors
- Produce virtual, erect, and diminished images regardless of the object's distance from the mirror.
- Images formed when the object is at infinity appear at focus F, regarded as point-sized.
- When the object is between infinity and the pole P of the mirror, the image is located between points P and F, still virtual and erect.
Results from Image Formation by Convex Mirrors
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Object at Infinity:
- Image at focus F, highly diminished and virtual.
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Object Between Infinity and Focus:
- Image located behind the mirror, diminished, and virtual.
Observations with Different Mirrors
- Plane mirrors may not show the full image of a large object.
- Concave mirrors and convex mirrors also vary in full-length image visibility based on the object distance.
Uses of Convex Mirrors
- Act as rear-view mirrors in vehicles, enhancing safety by providing a wider field of view.
- Erect images assist drivers in monitoring traffic, while the curved shape minimizes blind spots.
Sign Convention for Reflection
- New Cartesian Sign Convention is used for spherical mirrors.
- Pole (P) is the origin; distances measured from the pole:
- Positive to the right (x-axis) and above the principal axis (y-axis).
- Negative to the left (x-axis) and below the principal axis (y-axis).
Mirror Formula and Magnification
- The relationship between object distance (u), image distance (v), and focal length (f) is represented by the mirror formula:
- ( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} )
- Magnification (m) reflects the comparative height ratio of image to object:
- ( m = \frac{h'}{h} )
- It can also be calculated using distances: ( m = -\frac{v}{u} ).
Example Calculations
- Convex mirror with radius of curvature 3.00 m shows virtual image at 1.15 m behind the mirror.
- Concave mirror produces an inverted image at 37.5 cm with enlarged size based on object distance from the mirror.
Refraction of Light
- Refraction occurs when light passes from one medium to another, bending its path.
- Observations include the apparent displacement of objects under water or through glass slabs.
Laws of Refraction
- Incident, refracted rays, and the normal lie in the same plane.
- Snell’s Law relates the sine of angles of incidence and refraction, providing a constant for specific media:
- ( \frac{\sin i}{\sin r} = constant ) (refractive index).
Importance of Refractive Index
- Represents how the speed of light changes across different materials and quantifies the extent of bending during refraction.### Speed of Light in Different Media
- Light travels fastest in a vacuum at 3×10^8 m/s, with speed slightly reduced in air.
- The speed of light significantly decreases in denser materials like glass and water.
- The refractive index (n) compares the speed of light in two media: n_21 = v1/v2, where v1 is the speed in medium 1 and v2 is the speed in medium 2.
- Absolute refractive index of a medium (n_m) is defined as: n_m = c/v, where c is the speed of light in air.
Refractive Index Values
- Water has a refractive index (n_w) of 1.33, indicating light travels slower in water than in air.
- Crown glass has a refractive index (n_g) of 1.52.
- Examples of refractive indices from Table 9.3 include:
- Air: 1.0003
- Ice: 1.31
- Kerosene: 1.44
- Diamond: 2.42
- Optical density is not synonymous with mass density; a medium can be optically denser without being heavier.
Optical Density
- Optically denser media have higher refractive indices.
- Light travels faster in rarer media; it bends towards the normal when moving from rarer to denser media and bends away from the normal when moving from denser to rarer media.
Lenses
- Lenses are transparent materials with spherical surfaces that refract light.
- A convex lens (converging) is thicker in the middle and focuses light rays to a point.
- A concave lens (diverging) is thicker at the edges and appears to diverge light rays.
Principal Focus and Focal Length
- The principal focus of a convex lens is where parallel rays converge, while for a concave lens, it’s where rays appear to diverge.
- Focal length (f) is the distance from the optical center to the principal focus.
Image Formation by Lenses
- Lenses form images by refracting light based on the object’s position.
- Image characteristics vary by object distance from the lens, such as being real/inverted or virtual/erect.
Image Formation with Convex Lens
- Convex lens image formation varies according to object distance with characteristics summarized as follows:
- At infinity: Image at focus F2 (highly diminished, real, inverted).
- Beyond 2F1: Image between F2 and 2F2 (diminished, real, inverted).
- At 2F1: Image at 2F2 (same size, real, inverted).
- Between F1 and 2F1: Image beyond 2F2 (enlarged, real, inverted).
- At focus F1: Image at infinity (highly enlarged, real, inverted).
- Between F1 and O: Image on the same side (enlarged, virtual, erect).
Image Formation with Concave Lens
- Concave lenses consistently create virtual, erect, and diminished images regardless of object placement.
Ray Diagrams for Lenses
- Ray diagrams illustrate light behavior through lenses:
- Rays parallel to the principal axis converge at principal focus for a convex lens.
- Diverging rays from the principal focus appear parallel in a concave lens.
- Rays passing through the optical center continue without deviation.
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Description
This quiz covers the concepts of light, reflection, and refraction. It explores how light makes objects visible and the role of sunlight during the day.
