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Let's go through the questions step by step.

### Question 18:
- This question is about refraction and the apparent position of objects in water. When looking at an object underwater, the light rays bend as they pass from water to air, making the object appear at a different location than its actual position.
- In the diagram, the apparent position of the goldfish is given at point B. Since light bends away from the normal when it exits the water and enters air (because air has a lower refractive index than water), the actual position of the goldfish would be directly below point B, which is point C.

### Answer for 18:
The actual position of the goldfish is at point C.

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### Question 19:
- This question deals with the conditions for total internal reflection. Total internal reflection occurs when light attempts to move from a denser medium (higher refractive index) to a less dense medium (lower refractive index) and hits the boundary at an angle greater than the critical angle.
  
Let's analyze the statements:
1. **It occurs only when light travels from glass to air.**  
   This statement is incorrect because total internal reflection can occur in any situation where light travels from a medium with a higher refractive index to one with a lower refractive index, not just glass to air.

2. **It will never occur if light travels from air to glass.**  
   This is correct because, for total internal reflection to happen, light must travel from a medium with a higher refractive index to one with a lower refractive index. Since air has a lower refractive index than glass, total internal reflection cannot occur in this case.

3. **It will occur if light travels from a medium with a refractive index of 1.6 to air at an angle of incidence of 35°.**  
   To determine if total internal reflection occurs, we need to calculate the critical angle. The critical angle for a medium with a refractive index of 1.6 can be found using the equation:
   \[
   \sin \theta_c = \frac{n_{\text{air}}}{n_{\text{medium}}} = \frac{1}{1.6} \approx 0.625
   \]
   The critical angle, \( \theta_c \), is approximately 38.7°. Since 35° is less than the critical angle, total internal reflection would not occur.

### Answer for 19:
The correct choice is B. (2) only.

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### Question 20:
- This question involves the behavior of light rays as they pass through different media (air and water). When light enters an air bubble in water, it bends due to the change in refractive indices.
- Depending on the incident angles and the refractive indices, some rays may refract and some may undergo total internal reflection.

To solve this, the paths would need to be analyzed based on the angles of incidence and refraction rules.

Would you like a detailed analysis of Question 20 or more explanation on any of the previous questions? Here are some follow-up questions you might find useful:

1. What is the relationship between the refractive index and the speed of light in a medium?
2. How does Snell's Law apply to different refraction scenarios?
3. What conditions must be met for total internal reflection to occur?
4. How does the apparent depth of an object in water differ from its actual depth?
5. How do you calculate the critical angle for total internal reflection?

**Tip:** When solving refraction problems, always remember that light bends towards the normal when entering a denser medium and away from the normal when moving to a less dense medium.

18. The figure shows the apparent position of a goldfish as seen by an observer above water. Where is the actual position of the goldfish? 19. Which of the following statements about total internal reflection of light is/are correct? 20. Three light rays enter an air bubble in water. C is the center of the bubble. Which of the paths shown below is/are correct?

This page explains key concepts like the apparent position of objects underwater, conditions for total internal reflection, and how light behaves when passing through different media. Suitable for high school students in Grades 10-12.

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