# Another geomatric optics

• leolaw
In summary, the students in a physics lab found that the location where a bright object may be placed in order to produce an image three times the size of the object does depend on the object distance. However, the question asks why they don't have to redo the experiment even though the object distances are different. So, I think that the reason is that the magnification does depend on the object distance and if the students were to place the object at the same location but with a different distance, then the magnification would be different.

#### leolaw

Each student in a physics lab is assigned to find the location where a bright object may be placed in order that a concave mirror with raidius of curvature r = 40cm will produce an image three times the size of the object. Two students complete the assignment at different times using identical equipment, but when they compare notes later, they discover that their answers for the object distance are not the same. Explain why they do not necessarily need to repeat the lab, and justify you rresponse with a calculation.

Personally, I think that the magnifcation of the object DOES depend on the object distance right?
Because $$\frac{d_0 - f}{f} = \frac{d_0}{d_i}$$

$$d_i (d_0 - f) = f (d_0)$$

$$\frac{d_i}{d_o} = \frac{f}{d_0 - f}$$

$$\frac{-f}{d_0 - f} = m$$ and

$$\frac {-40}{d_0 - 40} = 3$$
which shows that the magnifcation does depend on the object distance.

So why do the question say that they do not have to redo the experiment even if the object distances are different?

m can take the value -3 or 3. Each gives an image 3 times the size of an object. Hope this helps.

but my point is that, you see $$d_0$$ in the equation? So the magnification does depend on the distance and indeed the object would only magnify three times the size of the original, if it is placed at that location. But the question asks that why you don't have to redo the experiment even though the distances of the object are different from the two student.
So I just don't get the reason of it

hohoho
nvm. you are right -3 and 3 will produce a different $$d_0$$, so if the two students are placing the obejct at exactly those location, then the magnification would be 3, even though one of them may be inverted.

Thx

## 1. What is geometric optics?

Geometric optics is a branch of optics that deals with the study of light as rays rather than waves. It focuses on the behavior of light as it travels through different materials and interacts with objects.

## 2. What is the difference between geometric optics and physical optics?

The main difference between geometric optics and physical optics is that geometric optics uses the ray model of light, while physical optics uses the wave model. Geometric optics is more concerned with the path of light rays and how they are affected by objects, while physical optics focuses on the wave properties of light such as diffraction and interference.

## 3. How is the reflection of light explained in geometric optics?

In geometric optics, the reflection of light is explained by the law of reflection, which states that the incident angle of a ray of light is equal to the reflected angle. This means that the direction of the reflected ray is symmetric to the incident ray with respect to the normal line of the reflecting surface.

## 4. What is the principle of least time in geometric optics?

The principle of least time, also known as Fermat's principle, states that light takes the path that requires the least time to travel from one point to another. This principle is used to explain the laws of reflection and refraction in geometric optics.

## 5. How is the refraction of light explained in geometric optics?

In geometric optics, the refraction of light is explained by Snell's law, which states that the ratio of the sines of the angles of incidence and refraction is equal to the ratio of the velocities of light in the two media. This law helps to predict the direction of the refracted ray when light passes through different materials with varying refractive indices.