中三物理熱學，光學問題

Since you did not give a description of the experiment, I could only provide general answers to your questions.

Q1: It is due to non-uniform heating of liquid. Liquid close to the heater has a higher temperature than that far from the heater. After the heater is turned off, heat flows from higher temperature region to lower temperature region, thus the thermometer still records a rise of temperature.

Q2: Slice has a large surface area, thus can absorbs heat more faster.

Q3: To increase the contact surface area between the thermometer bulb and the holder. The thermometer thus can measure more accurately the temperature of liquid.

Q4: Ice absorbs heat from the environment, thus it melts.

Q5: Is your lens a concave one or convex lens? This is the crucial point whether a real image can be formed.

2015-07-30 11:10:10 補充：
Amendment to answer to Q5:
Objects placed within the focal length of a lens (irrespective it is a convex or concave lens), the image formed is virtual. Hence, it cannot be captured on a screen.

2015-08-02 19:41:39 補充：
Yoour suppl question.
1. Mass of water boiled away= (2 - 0.5) kg = 1.5 kg
Energy wasted = 1.5 x 2.26x10^6 J = 3.39x10^6 J

2015-08-02 19:44:49 補充：
2. Time that the lighted bulb could last
= 3.39 x 10^6/100 s = 33900 s = 9 hr 25 min.