Math Problem Statement

A student places an object of height 35.0 cm at a distance of 30 cm in front of a concave mirror. The mirror has a focal length of magnitude 15.0 cm. What is the magnitude of the image distance, the image height, and the magnification? What type of image is formed, its size, and orientation?

Solution

To solve this problem involving a concave mirror, let’s analyze step by step:


Given:

  1. Object height (hoh_o) = 35.0cm35.0 \, \text{cm}
  2. Object distance (uu) = 30cm30 \, \text{cm} (in front of the mirror → taken as negative for concave mirrors)
  3. Focal length (ff) = 15.0cm15.0 \, \text{cm} (also negative since it’s a concave mirror)

Key Mirror Formula:

1f=1v+1u\frac{1}{f} = \frac{1}{v} + \frac{1}{u} Where:

  • ff = focal length
  • vv = image distance
  • uu = object distance

Rearranging for vv (image distance): 1v=1f1u\frac{1}{v} = \frac{1}{f} - \frac{1}{u}


Step 1: Calculate Image Distance (vv)

Substitute f=15cmf = -15 \, \text{cm} and u=30cmu = -30 \, \text{cm}: 1v=115130\frac{1}{v} = \frac{1}{-15} - \frac{1}{-30} Simplifying: 1v=115+130\frac{1}{v} = -\frac{1}{15} + \frac{1}{30} 1v=230+130\frac{1}{v} = -\frac{2}{30} + \frac{1}{30} 1v=130\frac{1}{v} = -\frac{1}{30} Taking the reciprocal: v=30cmv = -30 \, \text{cm}

Thus, the image distance is 30cm30 \, \text{cm} (negative indicates it’s on the same side as the object).


Step 2: Calculate Magnification (mm)

The magnification formula is: m=vum = -\frac{v}{u} Substitute v=30cmv = -30 \, \text{cm} and u=30cmu = -30 \, \text{cm}: m=3030=1m = -\frac{-30}{-30} = -1

So, the magnification is 1-1.


Step 3: Calculate Image Height (hih_i)

The image height can be calculated using: hi=mhoh_i = m \cdot h_o Substitute m=1m = -1 and ho=35.0cmh_o = 35.0 \, \text{cm}: hi=135.0=35.0cmh_i = -1 \cdot 35.0 = -35.0 \, \text{cm}

The image height is 35.0cm35.0 \, \text{cm} (negative indicates the image is inverted).


Step 4: Describe the Image

  • Type of image: Real
  • Size of the image: Same size as the object
  • Orientation of the image: Inverted

Final Answers:

  1. Magnitude of the image distance: 30cm30 \, \text{cm}
  2. Magnitude of the image height: 35.0cm35.0 \, \text{cm}
  3. Magnitude of the magnification: 11
  4. Type of image formed: Real
  5. Size of the image: Same size
  6. Orientation of the image: Inverted

Questions for Further Practice:

  1. What happens to the image if the object is placed at the focal point?
  2. How does the image change if the object distance is doubled?
  3. Derive the mirror formula mathematically.
  4. What are the characteristics of an image formed by a convex mirror?
  5. What type of image is formed if the magnification is +0.5+0.5?

Tip: Always use the correct sign convention (negative for concave mirrors) while solving problems involving spherical mirrors. Let me know if you want further clarifications!

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Math Problem Analysis

Mathematical Concepts

Optics
Concave Mirrors
Image Formation
Magnification

Formulas

Mirror formula: 1/f = 1/v + 1/u
Magnification formula: m = -v/u
Image height: h_i = m * h_o

Theorems

Mirror Formula
Sign Convention in Optics

Suitable Grade Level

Grade 10